Contents

Introduction

Prerequisites

Requirements

Components Used

Conventions

Background Information

Key Concepts

Example Scenarios

Understand DHCP

Current DHCP RFC References

DHCP Message Table

DHCPDISCOVER

DHCPOFFER

DHCPREQUEST

DHCPACK

DHCPNAK

DHCPDECLINE

DHCPINFORM

DHCPRELEASE

Renew the Lease

DHCP Packet Table

Client-Server Conversation for Client that Obtains DHCP Address Where Client and DHCP Server Reside on Same Subnet

Role of DHCP/BootP Relay Agent

Configure DHCP/BootP Relay Agent Feature on Cisco IOS® Router

Set Manual Bindings

How to Make DHCP Work on Secondary IP Segments

DHCP Client-Server Conversation with DHCP Relay Function

Process for a DHCP client to obtain an IP address

Pre-Execution Environment (PXE) Bootup DHCP Considerations

Understand and Troubleshoot DHCP With Sniffer Traces

Decode Sniffer Trace of DHCP Client and Server on Same LAN Segment

Network Topology Where DHCP Client and Server Reside on the Same LAN Segment

Decode Sniffer Trace of DHCP Client and Server Separated by a Router that is Configured as a DHCP Relay Agent

Sniffer-B Trace

Sniffer-A Trace

Troubleshoot DHCP When Client Workstations are Unable to Obtain DHCP Addresses

Case Study #1: DHCP Server on Same LAN Segment or VLAN as DHCP Client

Case Study #2: DHCP Server and DHCP Client are Separated by a Router Configured for DHCP/BootP Relay Agent Functionality

DHCP Server on Router Fails to Assign Adresses with a POOL EXHAUSTED Error

DHCP Troubleshoot Modules

Understand Where DHCP Problems Can Occur

Short List of Possible Causes of DHCP Problems:

A. Verify Physical Connectivity

C. Verify Issue as a Startup Problem

D. Verify Switch Port Configuration (STP Portfast and Other Commands)

E. Check for Known NIC Card or Catalyst Switch Issues

F. Distinguishwhether DHCP Clients Obtain IP Address on the Same Subnet or VLAN as DHCP Server

G. Verify Router DHCP/BootP Relay Configuration

H. Subscriber Identification (82) Option Turned On

I. DHCP Database Agent and DHCP Conflict Logging

J. Check CDP for IP Phone Connections

K. Remove Down SVI Disrupts DHCP Snooping Operation

L. Limited Broadcast Address

M. Debug DHCP With Router Debug Commands

Sample Output

Sample Output

Appendix A: Cisco IOS DHCP Sample Configuration

Related Information

Introduction

This document describes how to troubleshoot several common issues with Dynamic Host Configuration Protocol (DHCP) in a Cisco Catalyst switch network.

Prerequisites

Requirements

There are no specific prerequisites for this document.

Components Used

This document is not restricted to specific software and hardware versions.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

Conventions

Refer to  Cisco Technical Tips Conventions  for more information on document conventions.

Note: Only registered Cisco clients have access to internal bug reports.

Background Information

DHCP provides a mechanism through which computers that use Transmission Control Protocol/Internet Protocol (TCP/IP) can obtain protocol configuration parameters automatically through the network. DHCP is an open standard that was developed by the Dynamic Host Configuration-Working Group (DHC-WG) of the Internet Engineering Task Force (IETF).

DHCP is based on a client-server paradigm, in which the DHCP client, for example, a desktop computer, contacts a DHCP server for configuration parameters. The DHCP server is typically centrally located and operated by the network administrator. Because the server is run by a network administrator, DHCP clients can be reliably and dynamically configured with parameters appropriate to the current network architecture.

Most enterprise networks consist of multiple subnets divided into subnetworks referred to as Virtual LANS (VLANs), where routers route between the subnetworks. Since routers do not pass broadcasts by default, a DHCP server would be needed on each subnet unless the routers are configured to forward the DHCP broadcast with the DHCP Relay Agent feature.

Key Concepts

These are several key concepts of DHCP:

- DHCP clients initially have no configured IP address and must therefore send a broadcast request to obtain an IP address from a DHCP server.

- Routers, by default, do not forward broadcasts. It is necessary to accommodate client DHCP broadcast requests if the DHCP server is on another broadcast domain (Layer 3 (L3) network). This is performed by use of a DHCP Relay Agent.

- The Cisco router implementation of DHCP Relay is provided through interface-level ip helper commands

Example Scenarios

Scenario 1: Cisco Router Routing between DHCP Client and Server Networks

As configured in this diagram, interface Ethernet1 forwards the client broadcasted DHCPDISCOVER to 192.168.2.2 through interface Ethernet1. The DHCP server fulfills the request through unicast. No further configuration to the router is necessary in this example.

Routing Between DHCP Client and Server Networks

Scenario 2: Cisco Catalyst Switch with L3 Module Routing between DHCP Client and Server Networks

As configured in the diagram, interface VLAN20 forwards the client broadcasted DHCPDISCOVER to 192.168.2.2 through interface VLAN10. The DHCP server fulfills the request through unicast. No further configuration to the router is necessary in this example. The switch ports need to be configured as host ports and have Spanning-Tree Protocol (STP) portfast enabled, and trunking and channeling disabled.

L3 Module Route Between DHCP Client and Server Networks

Understand DHCP

DHCP was originally defined in Requests for Comments (RFCs) 1531  and has since been obsoleted by RFC 2131. DHCP is based on the Bootstrap Protocol (BootP), which is defined in RFC 951.

DHCP is used by workstations (hosts) to get initial configuration information, such as an IP address, subnet mask, and default gateway upon bootup. With DHCP, you do not have to manually configure each host With an IP address. Furthermore, if a host moves to a different IP subnet, it must use a different IP address than the one it previously used. DHCP takes care of this automatically. It allows the host to choose an IP address in the correct IP subnet.

Current DHCP RFC References

- RFC 2131 - DHCP

- RFC 2132 - DHCP Options and BootP Vendor Extensions

- RFC 1534 - Interoperation between DHCP and BootP

- RFC 1542 - Clarifications and Extensions for the BootP

- RFC 2241 - DHCP Options for Novell Directory Services

- RFC 2242 - Netware/IP Domain Name and Information

- RFC 2489 - Procedure for Defining New DHCP Options

DHCP uses a client-server model where one or more servers (DHCP servers) allocate IP addresses and other optional configuration parameters to clients (hosts) upon client bootup. These configuration parameters are leased by the server to the client for some specified amount of time. When a host boots up, the TCP/IP stack in the host transmits a broadcast (DHCPDISCOVER) message in order to gain an IP address and subnet mask, among other configuration parameters. This initiates an exchange between the DHCP server and the host. During this exchange, the client passes through these well-defined states:

1. Initializing

2. Selecting

3. Requesting

4. Bound

5. Renewing

6. Rebinding

To move between these states, the client and server can exchange the types of messages listed in the DHCP Message Table.

DHCP Message Table

Reference

Message

Description

0x01

DHCPDISCOVER

The client looks for available DHCP servers.

0x02

DHCPOFFER

The server response to the client DHCPDISCOVER.

0x03

DHCPREQUEST

The client broadcasts to the server, requests offered parameters from one server specifically, as defined in the packet.

0x04

DHCPDECLINE

The client-to-server communication, indicates that the network address is already in use.

0x05

DHCPACK

The server-to-client communication with configuration parameters, along with committed network address.

0x06

DHCPNAK

The server-to-client communication, refuses the request for configuration parameter.

0x07

DHCPRELEASE

The client-to-server communication, surrenders network address and cancels the remainder lease.

0x08

DHCPINFORM

The client-to-server communication, asks for only local configuration parameters that the client already has externally configured as an address.

DHCPDISCOVER

When a client boots up for the first time, it is said to be in the Initializing state, and transmits a DHCPDISCOVER message on its local physical subnet over User Datagram Protocol (UDP) port 67 (BootP server). Since the client has no way to know the subnet to which it belongs, the DHCPDISCOVER is an all subnets broadcast (destination IP address of 255.255.255.255), with a source IP address of 0.0.0.0. The source IP address is 0.0.0.0 since the client does not have a configured IP address. If a DHCP server exists on this local subnet and is configured and operates correctly, the DHCP server hears the broadcast and responds with a DHCPOFFER message. If a DHCP server does not exist on the local subnet, there must be a DHCP/BootP Relay Agent on this local subnet to forward the DHCPDISCOVER message to a subnet that contains a DHCP server.

This relay agent can either be a dedicated host (for example, Microsoft Windows Server), or a router (for example, a Cisco router configured with interface level IP helper statements).

DHCPOFFER

A DHCP server that receives a DHCPDISCOVER message can respond with a DHCPOFFER message on UDP port 68 (BootP client). The client receives the DHCPOFFER and moves into the Selecting state. This DHCPOFFER message contains initial configuration information for the client. For example, the DHCP server fills in the yiaddr field of the DHCPOFFER message with the requested IP address. The subnet mask and default gateway are specified in the options field, subnet mask and router options, respectively. Other common options in the DHCPOFFER message include IP Address lease time, renewal time, domain name server, and NetBIOS name server (WINS). The DHCP server sends the DHCPOFFER to the broadcast address but includes the client hardware address in the chaddr field of the offer, so the client knows that it is the intended destination. In the event that the DHCP server is not on the local subnet, the DHCP server sends the DHCPOFFER, as a unicast packet, on UDP port 67, back to the DHCP/BootP Relay Agent from which the DHCPDISCOVER came. The DHCP/BootP Relay Agent then either broadcasts or unicasts the DHCPOFFER on the local subnet on UDP port 68, which depends on the Broadcast flag set by the Bootp client.

DHCPREQUEST

After the client receives a DHCPOFFER, it responds with a DHCPREQUEST message, and indicates its intent to accept the parameters in the DHCPOFFER, and moves into the Requesting state. The client can receive multiple DHCPOFFER messages, one from each DHCP server that received the original DHCPDISCOVER message. The client chooses one DHCPOFFER and responds to that DHCP server only and, implicitly, declines all other DHCPOFFER messages. The client identifies the selected server after it populates the Server Identifier option field with the DHCP server IP address. The DHCPREQUEST is also a broadcast, so all DHCP servers that sent a DHCPOFFER see the DHCPREQUEST, and each knows whether its DHCPOFFER was accepted or declined. Any additional configuration options that the client requires are included in the options field of the DHCPREQUEST message. Even though the client has been offered an IP address, it sends the DHCPREQUEST message with a source IP address of 0.0.0.0. At this time, the client has not yet received verification that it is clear to use the IP address.

DHCPACK

After the DHCP server receives the DHCPREQUEST, it acknowledges the request with a DHCPACK message, and then completes the initialization process. The DHCPACK message has a source IP address of the DHCP server, and the destination address is once again a broadcast and contains all the parameters that the client requested in the DHCPREQUEST message. When the client receives the DHCPACK, it enters into the Bound state, and is now free to use the IP address to communicate on the network. Meanwhile, the DHCP server stores the lease in its database and uniquely identifies it with the client identifier or chaddr, and the associated IP address. Both the client and server use this combination of identifiers to refer to the lease. The client identifier is the MAC address of the device plus the media type.

Before the DHCP client begins to use the new address, the DHCP client must calculate the time parameters associated with a leased address, which are Lease Time (LT), Renewal Time (T1), and Rebind Time (T2). The typical default LT is 72 hours. You can use shorter lease times to conserve addresses, if needed.

DHCPNAK

If the selected server is unable to satisfy the DHCPREQUEST message, the DHCP server responds with a DHCPNAK message. When the client receives a DHCPNAK message or does not receive a response to a DHCPREQUEST message, the client restarts the configuration process when it goes into the Requesting state. The client retransmits the DHCPREQUEST at least four times within 60 seconds before it restarts the Initializing state.

DHCPDECLINE

The client receives the DHCPACK and, optionally, performs a final check on the parameters. The client performs this procedure when it sends the Address Resolution Protocol (ARP) requests for the IP address provided in the DHCPACK. If the client detects that the address is already in use when it receives a reply to the ARP request, the client sends a DHCPDECLINE message to the server and restarts the configuration process in the Requesting state.

DHCPINFORM

If a client has obtained a network address through some other means or has a manually configured IP address, a client workstation can use a DHCPINFORM request message to obtain other local configuration parameters, such as the domain name and Domain Name Servers (DNSs). When DHCP servers receive a DHCPINFORM message construct a DHCPACK message with any local configuration parameters appropriate for the client without a new IP address. This DHCPACK is sent unicast to the client.

DHCPRELEASE

A DHCP client can choose to relinquish its lease on a network address when it sends a DHCPRELEASE message to the DHCP server. The client identifies the lease to be released by the use of theclient identifierfield and network address in the DHCPRELEASE message. If you need to extend the current DHCP pool range, remove the current pool of addresses, and specify the new range of IP addresses under the DHCP pool. In order to remove specific IP addresses or a range of addresses that you want to be in the DHCP pool, use the command ip dhcp excluded-address.

Note: If devices use BOOTP, infinite length leases are shown in the DHCP bindings of routers.

Renew the Lease

Since the IP address is only leased from the server, the lease must be renewed from time to time. When one half of the lease time has expired (T1=0.5 x LT), the client tries to renew the lease. The client enters the Renewing state and sends a DHCPREQUEST message to the server, which holds the current lease. The server replies to the request to renew with a DHCPACK message if it agrees to renew the lease. The DHCPACK message contains the new lease and any new configuration parameters, in the event that any changes are made to the server during the time of the previous lease. If the client is unable to reach the server when it holds the lease for some reason, it attempts to renew the address from any DHCP server after the original DHCP server has not responded to the renewal requests within a time T2. The default value of T2 is ( 7/8 x LT). This means T1 < T2< LT.

If the client previously had a DHCP assigned IP address and it is restarted, the client specifically requests the previously leased IP address in a DHCPREQUEST packet. This DHCPREQUEST still has the source IP address as has 0.0.0.0 and the destination as the IP broadcast address 255.255.255.255.

When a client sends a DHCPREQUEST in the course of a reboot it must not fill in the server indentifier field and must instead fill in the requested IP address option field. Only RFC compliant clients populate the ciaddr field with the address requested instead of the DHCP option field. The DHCP server accepts either method. The behavior of the DHCP server depends on a number of factors, such as in the case of Windows NT DHCP servers, the version of the system that is used, as well as other factors, such as superscoping. If the DHCP server determines that the client can still use the requested IP address, it either remains silent or sends a DHCPACK for the DHCPREQUEST. If the server determines that the client cannot use the requested IP address, it sends a DHCPNACK back to the client. The client then moves to the Initializing state and sends a DHCPDISCOVER message.

Note: The DHCP server assigns the bottom IP address from a pool of IP addresses to the DHCP clients. When the lease of the bottom address expires, it is assigned to another client if it is requested. You cannot make any changes in the order DHCP addresses are assigned.

DHCP Packet Table

The DHCP message is variable in length and consists of fields listed in the DHCP Packet Table.

Note: This packet is a modified version of the original BootP packet.

Field

Bytes

Name

Description

op

1

OpCode

Identifies the packet as a request or reply: 1=BOOTREQUEST, 2=BOOTREPLY

htype

1

Hardware Type

Specifies the network hardware address type.

hlen

1

Hardware Length

Specifies the length hardware address length.

hops

1

Hops

The client sets the value to zero and the value increments if the request is forwarded across a router.

xid

4

Transaction ID

A random number that is chosen by the client. All DHCP messages exchanged for a given DHCP transaction use the ID (xid).

secs

2

Seconds

Specifies number of seconds since the DHCP process started.

flags

2

Flags

Indicates whether the message is broadcast or unicast.

ciaddr

4

Client IP address

Only used when client knows its IP address as in the case of the Bound, Renew, or Rebinding states.

yiaddr

4

Your IP address

If the client IP address is 0.0.0.0, the DHCP server places the offered client IP address in this field.

siaddr

4

Server IP address

If the client knows the IP address of the DHCP server, this field is populated with the DHCP server address. Otherwise, it is used in DHCPOFFER and DHCPACK from DHCP server.

giaddr

4

Router IP address (GI ADDR)

The Gateway IP address, filled in by the DHCP/BootP Relay Agent.

chaddr

16

Client MAC address

The DHCP client MAC address.

sname

64

Server name

The optional server host name.

file

128

Boot file name

The boot file name.

options

variable

Option parameters

The optional parameters that can be provided by the DHCP server. RFC 2132 gives all possible options.

Client-Server Conversation for Client that Obtains DHCP Address Where Client and DHCP Server Reside on Same Subnet

Packet Description

Source MAC Addr

Destination MAC Addr

Source IP Addr

Destination IP Addr

DHCPDISCOVER

Client

Broadcast

0.0.0.0

255.255.255.255

DHCPOFFER

DHCPServer

Broadcast

DHCPServer

255.255.255.255

DHCPREQUEST

Client

Broadcast

0.0.0.0

255.255.255.255

DHCPACK

DHCPServer

Broadcast

DHCPServer

255.255.255.255

Role of DHCP/BootP Relay Agent

Routers, by default, do not forward broadcast packets. Since DHCP client messages use the destination IP address of 255.255.255.255 (all Nets Broadcast), DHCP clients cannot send requests to a DHCP server on a different subnet unless the DHCP/BootP Relay Agent is configured on the router. The DHCP/BootP Relay Agent forwards DHCP requests on behalf of a DHCP client to the DHCP server. The DHCP/BootP Relay Agent appends its own IP address to the source IP address of the DHCP frames that go to the DHCP server. This allows the DHCP server to respond via unicast to the DHCP/BootP Relay Agent. The DHCP/BootP Relay Agent also populates the Gateway IP address field with the IP address of the interface on which the DHCP message is received from the client. The DHCP server uses the Gateway IP address field to determine the subnet from which the DHCPDISCOVER, DHCPREQUEST, or DHCPINFORM message originates.

Configure DHCP/BootP Relay Agent Feature on Cisco IOS® Router

The process to configure a Cisco router to forward BootP or DHCP requests is simple. You just have to configure an IP helper-address that points to the DHCP/BootP server or to the subnet broadcast address of the network the server is on.

Network Example:

DHCP/BootP Relay Agent

To forward the BootP/DHCP request from the client to the DHCP server, the ip helper-address interface command is used. The IP helper-address can be configured to forward any UDP broadcast based on UDP port number. By default, the IP helper-address forwards these UDP broadcasts:

- Trivial File Transfer Protocol (TFTP) (port 69)

- DNS (port 53), time service (port 37)

- NetBIOS name server (port 137)

- NetBIOS datagram server (port 138)

- Boot Protocol (DHCP/BootP) client and server datagrams (ports 67 and 68)

- Terminal Access Control Access Control System (TACACS) service (port 49)

- IEN-116 name service (port 42)

IP helper-addresses can direct UDP broadcasts to a unicast or broadcast IP address.However, do not use the IP helper-address to forward UDP broadcasts from one subnet to the broadcast address of another subnet, due to the large amount of broadcast flooding that can occur. Multiple IP helper-address entries on a single interface are supported as well:

version 12.0

service timestamps debug uptime

service timestamps log uptime

no service password-encryption

!

hostname router

!

!

!

interface Ethernet0

ip address 192.168.2.1 255.255.255.0

no ip directed-broadcast

!

interface Ethernet1

ip address 192.168.1.1 255.255.255.0

ip helper-address 192.168.2.2

ip helper-address 192.168.2.3

!--- IP helper-address pointing to DHCP server

no ip directed-broadcast

!

!

!

line con 0

exec-timeout 0 0

transport input none

line aux 0

line vty 0 4

login

!

end

Cisco routers do not support load balancing of DHCP servers that are configured as DHCP Relay Agents. Cisco routers forward the DHCPDISCOVER message to all the helper addresses mentioned for that interface. The use of two or more DHCP servers to serve a subnet only increases the DHCP traffic as the DHCPDISCOVER, DHCPOFFER, and DHCPREQUEST / DHCPDECLINE messages are exchanged between each pair of DHCP client and server.

Set Manual Bindings

There are two ways to set up manual bindings; one is for the Windows host, and the other is for non-Windows hosts. There are two different commands used to configure; one is for Microsoft DHCP clients, and the other is for non-Microsoft DHCP clients:  DHCPclient-identifier (manual binding - Microsoft DHCP clients) and DHCPhardware-address (manual binding - non-Microsoft DHCP clients). The reason for two different commands is that a PC that runs with Windows modifies its MACs, and a 01is added at the beginning of the address. These are the sample configurations:

- This is a configuration for Microsoft DHCP clients:

configure terminal

ip dhcp pool new_pool

host ip_address subnet_mask

client-identifier 01XXXXXXXXXXXX

!--- xxxxxx represents 48 bit MAC address prepended with 01

- This is a configuration for non-Microsoft DHCP clients:

configure terminal

ip dhcp pool new_pool

host ip_address subnet_mask

hardware-address XXXXXXXXXXXX

!--- xxxxxx represents 48 bit MAC address

How to Make DHCP Work on Secondary IP Segments

By default, DHCP has a limitation in that the reply packets are sent only if the request is received from the interface configured with the primary IP address. DHCP traffic uses the broadcast address. When the DHCP request is received by the router interface, it forwards it to the DHCP server (when IP helper-address is configured) with a source address of the primary IP configured on the interface to let the DHCP server know which IP pool it must use (for the client) in the DHCP reply packet.

There is no way for the router to know if the DHCP broadcast request comes from a device that is on the secondary IP network configured on the interface. As a workaround, sub-interface configuration (provided that the device connected to the router supports dot1q tagging) to separate the two subnets can be configured, so both of them get their correspondent IP addresses properly.

If the secondary address is the preferred way, there is another workaround, which is to enable the global configuration command ip dhcp smart-relay. This has a limitation in that it only uses the secondary IP to relay the DHCP request if there is no response from the DHCP server after three consecutive requests for the primary address pool.

DHCP Client-Server Conversation with DHCP Relay Function

The next table illustrates the process for a DHCP client to obtain an IP address from a DHCP server. This table is modeled after the previousConfigure DHCP/BootP Relay Agent Feature network diagram. Each numerical value in the diagram represents a packet that is described in this next table. Use this table to understand the the packet flow of DHCP client-server conversation. It also helps you to determine where problems occur.

Process for a DHCP client to obtain an IP address

Packet

Client IP Address

Server IP Address

GI Address

Packet Source MAC Address

Packet Source IP Address

Packet Destination MAC Address

Packet Destination IP Address

1. DHCPDISCOVER is sent from client.

0.0.0.0

0.0.0.0

0.0.0.0

0005.DCC9.C640

0.0.0.0

ffff.ffff.fffff (broadcast)

255.255.255.255

2. The router receives the DHCPDISCOVER on the E1 interface. The router recognizes that this packet is a DHCP UDP broadcast. The router now acts as a DHCP/BootP Relay Agent and fill in the Gateway IP address field with the incoming interface IP address, change the source IP address to an incoming interface IP address, and forward the request directly to the DHCP server.

0.0.0.0

0.0.0.0

192.168.1.1

Interface E2 MAC Address

192.168.1.1

MAC Address of DHCP Server

192.168.2.2

3. The DHCP server has received the DHCPDISCOVER and sends a DHCPOFFER to the DHCP Relay Agent.

192.168.1.2

192.168.2.2

192.168.1.1

MAC Address of DHCP Server

192.168.2.2

Interface E2 MAC Address

192.168.1.1

4. The DHCP Relay Agent receives a DHCPOFFER and forwards the DHCPOFFER broadcast on the local LAN.

192.168.1.2

192.168.2.2

192.168.1.1

Interface E1 MAC Address

192.168.1.1

ffff.ffff.ffff (broadcast)

255.255.255.255

5. DHCPREQUEST sent from client.

0.0.0.0

0.0.0.0

0.0.0.0

0005.DCC9.C640

0.0.0.0

ffff.ffff.fffff (broadcast)

255.255.255.255

6. The router receives the DHCPREQUEST on the E1 Interface. The router recognizes that this packet is DHCP UDP broadcast. The router now acts as a DHCP Relay Agent and fill in the Gateway IP address field with the sent interface IP Address, change the source IP address to an incoming interface IP address, and forward the request directly to the DHCP server.

0.0.0.0

0.0.0.0

192.168.1.1

Interface E2 MAC Address

192.168.1.1

MAC Address of DHCP Server

192.168.2.2

7. The DHCP server has received the DHCPREQUEST and sends a DHCPACK to the DHCP/BootP Relay Agent.

192.168.1.2

192.168.2.2

192.168.1.1

MAC Address of DHCP Server

192.168.2.2

Interface E2 MAC Address

192.168.1.1

8. The DHCP/BootP Relay Agent receives the DHCPACK and forwards the DHCPACK broadcast on the local LAN. The client accepts the ACK and use the client IP address.

192.168.1.2

192.168.2.2

192.168.1.1

Interface E1 MAC Address

192.168.1.1

ffff.ffff.ffff (broadcast)

255.255.255.255

Pre-Execution Environment (PXE) Bootup DHCP Considerations

Pre-Execution Environment (PXE) allows a workstation to boot from a server on a network prior to the boot of the system on the local hard drive. A network administrator does not have to physically visit the specific workstation and manually boot it. OS and other software, such as diagnostic programs, can be loaded onto the device from a server over the network. PXE environment uses DHCP to configure its IP address.

The DHCP/BootP Relay Agent configuration must be done on the router if the DHCP server is located on another routed segment of the network. The ip helper-address command on the local router interface must be configured. Refer to theConfigure DHCP/BootP Relay Agent Feature on Cisco IOS Routersection of this document for configuration information.

Understand and Troubleshoot DHCP With Sniffer Traces

Decode Sniffer Trace of DHCP Client and Server on Same LAN Segment

Network Topology Where DHCP Client and Server Reside on the Same LAN Segment

The sniffer trace example is comprised of six frames. These six frames illustrate a scenario in which the DHCP client and server reside on the same physical or logical segment. Use the next code example to you troubleshoot DHCP. It is important to match your sniffer trace to the traces in this example. There can be some differences compared to the next illustrated traces, but the general packet flow must be exactly the same. The packet trace trackss previous discussions of how DHCP works.

- - - - - - - - - - - - - - - - - - - - Frame 1 - DHCPDISCOVER - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

1[0.0.0.0] [255.255.255.255] 618 0:01:26.810 0.575.244 05/07/2001 11:52:03 AM DHCP: Request,

Message type: DHCP Discover

DLC: ----- DLC Header -----

DLC:

DLC: Frame 1arrived at 11:52:03.8106; frame size is 618 (026A hex) bytes.

DLC: Destination = BROADCAST FFFFFFFFFFFF, Broadcast

DLC: Source = Station 0005DCC9C640

DLC: Ethertype = 0800 (IP)

DLC:

IP: ----- IP Header -----

IP:

IP: Version = 4, header length = 20 bytes

IP: Type of service = 00

IP: 000. .... = routine

IP: ...0 .... = normal delay

IP: .... 0... = normal throughput

IP: .... .0.. = normal reliability

IP: .... ..0. = ECT bit - transport protocol will ignore the CE bit

IP: .... ...0 = CE bit - no congestion

IP: Total length = 604 bytes

IP: Identification = 9

IP: Flags = 0X

IP: .0.. .... = may fragment

IP: ..0. .... = last fragment

IP: Fragment offset = 0 bytes

IP: Time to live = 255 seconds/hops

IP: Protocol = 17 (UDP)

IP: Header checksum = B988 (correct)

IP: Source address = [0.0.0.0]

IP: Destination address = [255.255.255.255]

IP: No options

IP:

UDP: ----- UDP Header -----

UDP:

UDP: Source port = 68 (BootPc/DHCP)

UDP: Destination port = 67 (BootPs/DHCP)

UDP: Length = 584

UDP: No checksum

UDP: [576 byte(s) of data]

UDP:

DHCP: ----- DHCP Header -----

DHCP:

DHCP: Boot record type = 1 (Request)

DHCP: Hardware address type = 1 (10Mb Ethernet)

DHCP: Hardware address length = 6 bytes

DHCP:

DHCP: Hops = 0

DHCP: Transaction id = 00000882

DHCP: Elapsed boot time = 0 seconds

DHCP: Flags = 8000

DHCP: 1... .... .... .... = Broadcast IP datagrams

DHCP: Client self-assigned IP address = [0.0.0.0]

DHCP: Client IP address = [0.0.0.0]

DHCP: Next Server to use in bootstrap = [0.0.0.0]

DHCP: Relay Agent = [0.0.0.0]

DHCP: Client hardware address = 0005DCC9C640

DHCP:

DHCP: Host name = ""

DHCP: Boot file name = ""

DHCP:

DHCP: Vendor Information tag = 63825363

DHCP: Message Type = 1 (DHCP Discover)

DHCP: Maximum message size = 1152

DHCP: Client identifier = 00636973636F2D303030352E646363392E633634302D564C31

DHCP: Parameter Request List: 7 entries

DHCP: 1 = Client's subnet mask

DHCP: 66 = TFTP Option

DHCP: 6 = Domain name server

DHCP: 3 = Routers on the client's subnet

DHCP: 67 = Boot File Option

DHCP: 12 = Host name server

DHCP: 150 = Unknown Option

DHCP: Class identifier = 646F63736973312E30

DHCP: Option overload =3 (File and Sname fields hold options)

DHCP:

- - - - - - - - - - - - - - - - - - - - Frame 2 - DHCPOFFER - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

2[192.168.1.1] [255.255.255.255] 331 0:01:26.825 0.015.172 05/07/2001 11:52:03 AM DHCP: Reply,

Message type: DHCP Offer

DLC: ----- DLC Header -----

DLC:

DLC: Frame 2 arrived at 11:52:03.8258; frame size is 331 (014B hex) bytes.

DLC: Destination = BROADCAST FFFFFFFFFFFF, Broadcast

DLC: Source = Station 0005DCC42484

DLC: Ethertype = 0800 (IP)

DLC:

IP: ----- IP Header -----

IP:

IP: Version = 4, header length = 20 bytes

IP: Type of service = 00

IP: 000. .... = routine

IP: ...0 .... = normal delay

IP: .... 0... = normal throughput

IP: .... .0.. = normal reliability

IP: .... ..0. = ECT bit - transport protocol will ignore the CE bit

IP: .... ...0 = CE bit - no congestion

IP: Total length = 317 bytes

IP: Identification = 5

IP: Flags = 0X

IP: .0.. .... = may fragment

IP: ..0. .... = last fragment

IP: Fragment offset = 0 bytes

IP: Time to live = 255 seconds/hops

IP: Protocol = 17 (UDP)

IP: Header checksum = F901 (correct)

IP: Source address = [192.168.1.1]

IP: Destination address = [255.255.255.255]

IP: No options

IP:

UDP: ----- UDP Header -----

UDP:

UDP: Source port = 67 (BootPs/DHCP)

UDP: Destination port = 68 (BootPc/DHCP)

UDP: Length = 297

UDP: No checksum

UDP: [289 byte(s) of data]

UDP:

DHCP: ----- DHCP Header -----

DHCP:

DHCP: Boot record type = 2 (Reply)

DHCP: Hardware address type = 1 (10Mb Ethernet)

DHCP: Hardware address length = 6 bytes

DHCP:

DHCP: Hops = 0

DHCP: Transaction id = 00000882

DHCP: Elapsed boot time = 0 seconds

DHCP: Flags = 8000

DHCP: 1... .... .... .... = Broadcast IP datagrams

DHCP: Client self-assigned IP address = [0.0.0.0]

DHCP: Client IP address = [192.168.1.2]

DHCP: Next Server to use in bootstrap = [0.0.0.0]

DHCP: Relay Agent = [0.0.0.0]

DHCP: Client hardware address = 0005DCC9C640

DHCP:

DHCP: Host name = ""

DHCP: Boot file name = ""

DHCP:

DHCP: Vendor Information tag = 63825363

DHCP: Message Type = 2 (DHCP Offer)

DHCP: Server IP address = [192.168.1.1]

DHCP: Request IP address lease time = 85535 (seconds)

DHCP: Address Renewal interval = 42767 (seconds)

DHCP: Address Rebinding interval = 74843 (seconds)

DHCP: Subnet mask = [255.255.255.0]

DHCP: Domain Name Server address = [192.168.1.3]

DHCP: Domain Name Server address = [192.168.1.4]

DHCP: Gateway address = [192.168.1.1]

DHCP:

- - - - - - - - - - - - - - - - - - - - Frame 3 - DHCPREQUEST - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

3[0.0.0.0] [255.255.255.255] 618 0:01:26.829 0.003.586 05/07/2001 11:52:03 AM DHCP: Request,

Message type: DHCP Request

DLC: ----- DLC Header -----

DLC:

DLC: Frame 56 arrived at 11:52:03.8294; frame size is 618 (026A hex) bytes.

DLC: Destination = BROADCAST FFFFFFFFFFFF, Broadcast

DLC: Source = Station 0005DCC9C640

DLC: Ethertype = 0800 (IP)

DLC:

IP: ----- IP Header -----

IP:

IP: Version = 4, header length = 20 bytes

IP: Type of service = 00

IP: 000. .... = routine

IP: ...0 .... = normal delay

IP: .... 0... = normal throughput

IP: .... .0.. = normal reliability

IP: .... ..0. = ECT bit - transport protocol will ignore the CE bit

IP: .... ...0 = CE bit - no congestion

IP: Total length = 604 bytes

IP: Identification = 10

IP: Flags = 0X

IP: .0.. .... = may fragment

IP: ..0. .... = last fragment

IP: Fragment offset = 0 bytes

IP: Time to live = 255 seconds/hops

IP: Protocol = 17 (UDP)

IP: Header checksum = B987 (correct)

IP: Source address = [0.0.0.0]

IP: Destination address = [255.255.255.255]

IP: No options

IP:

UDP: ----- UDP Header -----

UDP:

UDP: Source port = 68 (BootPc/DHCP)

UDP: Destination port = 67 (BootPs/DHCP)

UDP: Length = 584

UDP: No checksum

UDP: [576 byte(s) of data]

UDP:

DHCP: ----- DHCP Header -----

DHCP:

DHCP: Boot record type = 1 (Request)

DHCP: Hardware address type = 1 (10Mb Ethernet)

DHCP: Hardware address length = 6 bytes

DHCP:

DHCP: Hops = 0

DHCP: Transaction id = 00000882

DHCP: Elapsed boot time = 0 seconds

DHCP: Flags = 8000

DHCP: 1... .... .... .... = Broadcast IP datagrams

DHCP: Client self-assigned IP address = [0.0.0.0]

DHCP: Client IP address = [0.0.0.0]

DHCP: Next Server to use in bootstrap = [0.0.0.0]

DHCP: Relay Agent = [0.0.0.0]

DHCP: Client hardware address = 0005DCC9C640

DHCP:

DHCP: Host name = ""

DHCP: Boot file name = ""

DHCP:

DHCP: Vendor Information tag = 63825363

DHCP: Message Type = 3 (DHCP Request)

DHCP: Maximum message size = 1152

DHCP: Client identifier = 00636973636F2D303030352E646363392E633634302D564C31

DHCP: Server IP address = [192.168.1.1]

DHCP: Request specific IP address = [192.168.1.2]

DHCP: Request IP address lease time = 85535 (seconds)

DHCP: Parameter Request List: 7 entries

DHCP: 1 = Client's subnet mask

DHCP: 66 = TFTP Option

DHCP: 6 = Domain name server

DHCP: 3 = Routers on the client's subnet

DHCP: 67 = Boot File Option

DHCP: 12 = Host name server

DHCP: 150 = Unknown Option

DHCP: Class identifier = 646F63736973312E30

DHCP: Option overload =3 (File and Sname fields hold options)

DHCP:

- - - - - - - - - - - - - - - - - - - - Frame 4 - DHCPACK - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

4[192.168.1.1] [255.255.255.255] 331 0:01:26.844 0.014.658 05/07/2001 11:52:03 AM DHCP: Reply,

Message type: DHCP Ack

DLC: ----- DLC Header -----

DLC:

DLC: Frame 57 arrived at 11:52:03.8440; frame size is 331 (014B hex) bytes.

DLC: Destination = BROADCAST FFFFFFFFFFFF, Broadcast

DLC: Source = Station 0005DCC42484

DLC: Ethertype = 0800 (IP)

DLC:

IP: ----- IP Header -----

IP:

IP: Version = 4, header length = 20 bytes

IP: Type of service = 00

IP: 000. .... = routine

IP: ...0 .... = normal delay

IP: .... 0... = normal throughput

IP: .... .0.. = normal reliability

IP: .... ..0. = ECT bit - transport protocol will ignore the CE bit

IP: .... ...0 = CE bit - no congestion

IP: Total length = 317 bytes

IP: Identification = 6

IP: Flags = 0X

IP: .0.. .... = may fragment

IP: ..0. .... = last fragment

IP: Fragment offset = 0 bytes

IP: Time to live = 255 seconds/hops

IP: Protocol = 17 (UDP)

IP: Header checksum = F900 (correct)

IP: Source address = [192.168.1.1]

IP: Destination address = [255.255.255.255]

IP: No options

IP:

UDP: ----- UDP Header -----

UDP:

UDP: Source port = 67 (BootPs/DHCP)

UDP: Destination port = 68 (BootPc/DHCP)

UDP: Length = 297

UDP: No checksum

UDP: [289 byte(s) of data]

UDP:

DHCP: ----- DHCP Header -----

DHCP:

DHCP: Boot record type = 2 (Reply)

DHCP: Hardware address type = 1 (10Mb Ethernet)

DHCP: Hardware address length = 6 bytes

DHCP:

DHCP: Hops = 0

DHCP: Transaction id = 00000882

DHCP: Elapsed boot time = 0 seconds

DHCP: Flags = 8000

DHCP: 1... .... .... .... = Broadcast IP datagrams

DHCP: Client self-assigned IP address = [0.0.0.0]

DHCP: Client IP address = [192.168.1.2]

DHCP: Next Server to use in bootstrap = [0.0.0.0]

DHCP: Relay Agent = [0.0.0.0]

DHCP: Client hardware address = 0005DCC9C640

DHCP:

DHCP: Host name = ""

DHCP: Boot file name = ""

DHCP:

DHCP: Vendor Information tag = 63825363

DHCP: Message Type = 5 (DHCP Ack)

DHCP: Server IP address = [192.168.1.1]

DHCP: Request IP address lease time = 86400 (seconds)

DHCP: Address Renewal interval = 43200 (seconds)

DHCP: Address Rebinding interval = 75600 (seconds)

DHCP: Subnet mask = [255.255.255.0]

DHCP: Domain Name Server address = [192.168.1.3]

DHCP: Domain Name Server address = [192.168.1.4]

DHCP: Gateway address = [192.168.1.1]

DHCP:

- - - - - - - - - - - - - - - - - - - - Frame 5 - ARP - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

5 0005DCC9C640 Broadcast 60 0:01:26.846 0.002.954 05/07/2001 11:52:03 AM ARP: R PA=[192.168.1.2]

HA=0005DCC9C640 PRO=IP

DLC: ----- DLC Header -----

DLC:

DLC: Frame 58 arrived at 11:52:03.8470; frame size is 60 (003C hex) bytes.

DLC: Destination = BROADCAST FFFFFFFFFFFF, Broadcast

DLC: Source = Station 0005DCC9C640

DLC: Ethertype = 0806 (ARP)

DLC:

ARP: ----- ARP/RARP frame -----

ARP:

ARP: Hardware type = 1 (10Mb Ethernet)

ARP: Protocol type = 0800 (IP)

ARP: Length of hardware address = 6 bytes

ARP: Length of protocol address = 4 bytes

ARP: Opcode 2 (ARP reply)

ARP: Sender's hardware address = 0005DCC9C640

ARP: Sender's protocol address = [192.168.1.2]

ARP: Target hardware address = FFFFFFFFFFFF

ARP: Target protocol address = [192.168.1.2]

ARP:

ARP: 18 bytes frame padding

ARP:

- - - - - - - - - - - - - - - - - - - - Frame 6 - ARP - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

6 0005DCC9C640 Broadcast 60 0:01:27.355 0.508.778 05/07/2001 11:52:04 AM ARP: R PA=[192.168.1.2]

HA=0005DCC9C640 PRO=IP

DLC: ----- DLC Header -----

DLC:

DLC: Frame 59 arrived at 11:52:04.3557; frame size is 60 (003C hex) bytes.

DLC: Destination = BROADCAST FFFFFFFFFFFF, Broadcast

DLC: Source = Station 0005DCC9C640

DLC: Ethertype = 0806 (ARP)

DLC:

ARP: ----- ARP/RARP frame -----

ARP:

ARP: Hardware type = 1 (10Mb Ethernet)

ARP: Protocol type = 0800 (IP)

ARP: Length of hardware address = 6 bytes

ARP: Length of protocol address = 4 bytes

ARP: Opcode 2 (ARP reply)

ARP: Sender's hardware address = 0005DCC9C640

ARP: Sender's protocol address = [192.168.1.2]

ARP: Target hardware address = FFFFFFFFFFFF

ARP: Target protocol address = [192.168.1.2]

ARP:

ARP: 18 bytes frame padding

ARP:

Decode Sniffer Trace of DHCP Client and Server Separated by a Router that is Configured as a DHCP Relay Agent

Sniffer-B Trace

- - - - - - - - - - - - - - - - - - - - Frame 1 - DHCPDISCOVER - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

1 [0.0.0.0] [255.255.255.255] 618 0:02:05.759 0.025.369 05/31/2001 06:53:04 AM DHCP: Request,

Message type: DHCP Discover

DLC: ----- DLC Header -----

DLC:

DLC: Frame 124 arrived at 06:53:04.2043; frame size is 618 (026A hex) bytes.

DLC: Destination = BROADCAST FFFFFFFFFFFF, Broadcast

DLC: Source = Station 0005DCF2C441

DLC: Ethertype = 0800 (IP)

DLC:

IP: ----- IP Header -----

IP:

IP: Version = 4, header length = 20 bytes

IP: Type of service = 00

IP: 000. .... = routine

IP: ...0 .... = normal delay

IP: .... 0... = normal throughput

IP: .... .0.. = normal reliability

IP: .... ..0. = ECT bit - transport protocol will ignore the CE bit

IP: .... ...0 = CE bit - no congestion

IP: Total length = 604 bytes

IP: Identification = 183

IP: Flags = 0X

IP: .0.. .... = may fragment

IP: ..0. .... = last fragment

IP: Fragment offset = 0 bytes

IP: Time to live = 255 seconds/hops

IP: Protocol = 17 (UDP)

IP: Header checksum = B8DA (correct)

IP: Source address = [0.0.0.0]

IP: Destination address = [255.255.255.255]

IP: No options

IP:

UDP: ----- UDP Header -----

UDP:

UDP: Source port = 68 (BootPc/DHCP)

UDP: Destination port = 67 (BootPs/DHCP)

UDP: Length = 584

UDP: No checksum

UDP: [576 byte(s) of data]

UDP:

DHCP: ----- DHCP Header -----

DHCP:

DHCP: Boot record type = 1 (Request)

DHCP: Hardware address type = 1 (10Mb Ethernet)

DHCP: Hardware address length = 6 bytes

DHCP:

DHCP: Hops = 0

DHCP: Transaction id = 00001425

DHCP: Elapsed boot time = 0 seconds

DHCP: Flags = 8000

DHCP: 1... .... .... .... = Broadcast IP datagrams

DHCP: Client self-assigned IP address = [0.0.0.0]

DHCP: Client IP address = [0.0.0.0]

DHCP: Next Server to use in bootstrap = [0.0.0.0]

DHCP: Relay Agent = [0.0.0.0]

DHCP: Client hardware address = 0005DCF2C441

DHCP:

DHCP: Host name = ""

DHCP: Boot file name = ""

DHCP:

DHCP: Vendor Information tag = 63825363

DHCP: Message Type = 1 (DHCP Discover)

DHCP: Maximum message size = 1152

DHCP: Client identifier = 00636973636F2D303065302E316566322E633434312D4574302F30

DHCP: Parameter Request List: 7 entries

DHCP: 1 = Client's subnet mask

DHCP: 6 = Domain name server

DHCP: 15 = Domain name

DHCP: 44 = NetBIOS over TCP/IP name server

DHCP: 3 = Routers on the client's subnet

DHCP: 33 = Static route

DHCP: 150 = Unknown Option

DHCP: Class identifier = 646F63736973312E30

DHCP: Option overload =3 (File and Sname fields hold options)

DHCP:

- - - - - - - - - - - - - - - - - - - - Frame 2 - DHCPOFFER - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summaryr

125 [192.168.1.1] [255.255.255.255] 347 0:02:05.772 0.012.764 05/31/2001 06:53:04 AM DHCP: Reply,

Message type: DHCP Offer

DLC: ----- DLC Header -----

DLC:

DLC: Frame 125 arrived at 06:53:04.2171; frame size is 347 (015B hex) bytes.

DLC: Destination = BROADCAST FFFFFFFFFFFF, Broadcast

DLC: Source = Station 003094248F71

DLC: Ethertype = 0800 (IP)

DLC:

IP: ----- IP Header -----

IP:

IP: Version = 4, header length = 20 bytes

IP: Type of service = 00

IP: 000. .... = routine

IP: ...0 .... = normal delay

IP: .... 0... = normal throughput

IP: .... .0.. = normal reliability

IP: .... ..0. = ECT bit - transport protocol will ignore the CE bit

IP: .... ...0 = CE bit - no congestion

IP: Total length = 333 bytes

IP: Identification = 45

IP: Flags = 0X

IP: .0.. .... = may fragment

IP: ..0. .... = last fragment

IP: Fragment offset = 0 bytes

IP: Time to live = 255 seconds/hops

IP: Protocol = 17 (UDP)

IP: Header checksum = F8C9 (correct)

IP: Source address = [192.168.1.1]

IP: Destination address = [255.255.255.255]

IP: No options

IP:

UDP: ----- UDP Header -----

UDP:

UDP: Source port = 67 (BootPs/DHCP)

UDP: Destination port = 68 (BootPc/DHCP)

UDP: Length = 313

UDP: Checksum = 8517 (correct)

UDP: [305 byte(s) of data]

UDP:

DHCP: ----- DHCP Header -----

DHCP:

DHCP: Boot record type = 2 (Reply)

DHCP: Hardware address type = 1 (10Mb Ethernet)

DHCP: Hardware address length = 6 bytes

DHCP:

DHCP: Hops = 0

DHCP: Transaction id = 00001425

DHCP: Elapsed boot time = 0 seconds

DHCP: Flags = 8000

DHCP: 1... .... .... .... = Broadcast IP datagrams

DHCP: Client self-assigned IP address = [0.0.0.0]

DHCP: Client IP address = [192.168.1.2]

DHCP: Next Server to use in bootstrap = [0.0.0.0]

DHCP: Relay Agent = [192.168.1.1]

DHCP: Client hardware address = 0005DCF2C441

DHCP:

DHCP: Host name = ""

DHCP: Boot file name = ""

DHCP:

DHCP: Vendor Information tag = 63825363

DHCP: Message Type = 2 (DHCP Offer)

DHCP: Server IP address = [192.168.2.2]

DHCP: Request IP address lease time = 99471 (seconds)

DHCP: Address Renewal interval = 49735 (seconds)

DHCP: Address Rebinding interval = 87037 (seconds)

DHCP: Subnet mask = [255.255.255.0]

DHCP: Domain Name Server address = [192.168.10.1]

DHCP: Domain Name Server address = [192.168.10.2]

DHCP: NetBIOS Server address = [192.168.10.1]

DHCP: NetBIOS Server address = [192.168.10.3]

DHCP: Domain name = "cisco.com"

DHCP:

- - - - - - - - - - - - - - - - - - - - Frame 3 - DHCPREQUEST - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

3 [0.0.0.0] [255.255.255.255] 618 0:02:05.774 0.002.185 05/31/2001 06:53:04 AM DHCP: Request,

Message type: DHCP Request

DLC: ----- DLC Header -----

DLC:

DLC: Frame 126 arrived at 06:53:04.2193; frame size is 618 (026A hex) bytes.

DLC: Destination = BROADCAST FFFFFFFFFFFF, Broadcast

DLC: Source = Station Cisc14F2C441

DLC: Ethertype = 0800 (IP)

DLC:

IP: ----- IP Header -----

IP:

IP: Version = 4, header length = 20 bytes

IP: Type of service = 00

IP: 000. .... = routine

IP: ...0 .... = normal delay

IP: .... 0... = normal throughput

IP: .... .0.. = normal reliability

IP: .... ..0. = ECT bit - transport protocol will ignore the CE bit

IP: .... ...0 = CE bit - no congestion

IP: Total length = 604 bytes

IP: Identification = 184

IP: Flags = 0X

IP: .0.. .... = may fragment

IP: ..0. .... = last fragment

IP: Fragment offset = 0 bytes

IP: Time to live = 255 seconds/hops

IP: Protocol = 17 (UDP)

IP: Header checksum = B8D9 (correct)

IP: Source address = [0.0.0.0]

IP: Destination address = [255.255.255.255]

IP: No options

IP:

UDP: ----- UDP Header -----

UDP:

UDP: Source port = 68 (BootPc/DHCP)

UDP: Destination port = 67 (BootPs/DHCP)

UDP: Length = 584

UDP: No checksum

UDP: [576 byte(s) of data]

UDP:

DHCP: ----- DHCP Header -----

DHCP:

DHCP: Boot record type = 1 (Request)

DHCP: Hardware address type = 1 (10Mb Ethernet)

DHCP: Hardware address length = 6 bytes

DHCP:

DHCP: Hops = 0

DHCP: Transaction id = 00001425

DHCP: Elapsed boot time = 0 seconds

DHCP: Flags = 8000

DHCP: 1... .... .... .... = Broadcast IP datagrams

DHCP: Client self-assigned IP address = [0.0.0.0]

DHCP: Client IP address = [0.0.0.0]

DHCP: Next Server to use in bootstrap = [0.0.0.0]

DHCP: Relay Agent = [0.0.0.0]

DHCP: Client hardware address = 0005DCF2C441

DHCP:

DHCP: Host name = ""

DHCP: Boot file name = ""

DHCP:

DHCP: Vendor Information tag = 63825363

DHCP: Message Type = 3 (DHCP Request)

DHCP: Maximum message size = 1152

DHCP: Client identifier = 00636973636F2D303065302E316566322E633434312D4574302F30

DHCP: Server IP address = [192.168.2.2]

DHCP: Request specific IP address = [192.168.1.2]

DHCP: Request IP address lease time = 99471 (seconds)

DHCP: Parameter Request List: 7 entries

DHCP: 1 = Client's subnet mask

DHCP: 6 = Domain name server

DHCP: 15 = Domain name

DHCP: 44 = NetBIOS over TCP/IP name server

DHCP: 3 = Routers on the client's subnet

DHCP: 33 = Static route

DHCP: 150 = Unknown Option

DHCP: Class identifier = 646F63736973312E30

DHCP: Option overload =3 (File and Sname fields hold options)

DHCP:

- - - - - - - - - - - - - - - - - - - - Frame 4 - DHCPACK - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

4 [192.168.1.1] [255.255.255.255] 347 0:02:05.787 0.012.875 05/31/2001 06:53:04 AM DHCP: Reply,

Message type: DHCP Ack

DLC: ----- DLC Header -----

DLC:

DLC: Frame 127 arrived at 06:53:04.2321; frame size is 347 (015B hex) bytes.

DLC: Destination = BROADCAST FFFFFFFFFFFF, Broadcast

DLC: Source = Station 003094248F71

DLC: Ethertype = 0800 (IP)

DLC:

IP: ----- IP Header -----

IP:

IP: Version = 4, header length = 20 bytes

IP: Type of service = 00

IP: 000. .... = routine

IP: ...0 .... = normal delay

IP: .... 0... = normal throughput

IP: .... .0.. = normal reliability

IP: .... ..0. = ECT bit - transport protocol will ignore the CE bit

IP: .... ...0 = CE bit - no congestion

IP: Total length = 333 bytes

IP: Identification = 47

IP: Flags = 0X

IP: .0.. .... = may fragment

IP: ..0. .... = last fragment

IP: Fragment offset = 0 bytes

IP: Time to live = 255 seconds/hops

IP: Protocol = 17 (UDP)

IP: Header checksum = F8C7 (correct)

IP: Source address = [192.168.1.1]

IP: Destination address = [255.255.255.255]

IP: No options

IP:

UDP: ----- UDP Header -----

UDP:

UDP: Source port = 67 (BootPs/DHCP)

UDP: Destination port = 68 (BootPc/DHCP)

UDP: Length = 313

UDP: Checksum = 326F (correct)

UDP: [305 byte(s) of data]

UDP:

DHCP: ----- DHCP Header -----

DHCP:

DHCP: Boot record type = 2 (Reply)

DHCP: Hardware address type = 1 (10Mb Ethernet)

DHCP: Hardware address length = 6 bytes

DHCP:

DHCP: Hops = 0

DHCP: Transaction id = 00001425

DHCP: Elapsed boot time = 0 seconds

DHCP: Flags = 8000

DHCP: 1... .... .... .... = Broadcast IP datagrams

DHCP: Client self-assigned IP address = [0.0.0.0]

DHCP: Client IP address = [192.168.1.2]

DHCP: Next Server to use in bootstrap = [0.0.0.0]

DHCP: Relay Agent = [192.168.1.1]

DHCP: Client hardware address = 0005DCF2C441

DHCP:

DHCP: Host name = ""

DHCP: Boot file name = ""

DHCP:

DHCP: Vendor Information tag = 63825363

DHCP: Message Type = 5 (DHCP Ack)

DHCP: Server IP address = [192.168.2.2]

DHCP: Request IP address lease time = 172800 (seconds)

DHCP: Address Renewal interval = 86400 (seconds)

DHCP: Address Rebinding interval = 151200 (seconds)

DHCP: Subnet mask = [255.255.255.0]

DHCP: Domain Name Server address = [192.168.10.1]

DHCP: Domain Name Server address = [192.168.10.2]

DHCP: NetBIOS Server address = [192.168.10.1]

DHCP: NetBIOS Server address = [192.168.10.3]

DHCP: Domain name = "cisco.com"

DHCP:

- - - - - - - - - - - - - - - - - - - - Frame 5 - ARP - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

5 Cisc14F2C441 Broadcast 60 0:02:05.798 0.011.763 05/31/2001 06:53:04 AM ARP: R PA=[192.168.1.2]

HA=Cisc14F2C441 PRO=IP

DLC: ----- DLC Header -----

DLC:

DLC: Frame 128 arrived at 06:53:04.2439; frame size is 60 (003C hex) bytes.

DLC: Destination = BROADCAST FFFFFFFFFFFF, Broadcast

DLC: Source = Station Cisc14F2C441

DLC: Ethertype = 0806 (ARP)

DLC:

ARP: ----- ARP/RARP frame -----

ARP:

ARP: Hardware type = 1 (10Mb Ethernet)

ARP: Protocol type = 0800 (IP)

ARP: Length of hardware address = 6 bytes

ARP: Length of protocol address = 4 bytes

ARP: Opcode 2 (ARP reply)

ARP: Sender's hardware address = 00E01EF2C441

ARP: Sender's protocol address = [192.168.1.2]

ARP: Target hardware address = FFFFFFFFFFFF

ARP: Target protocol address = [192.168.1.2]

ARP:

ARP: 18 bytes frame padding

ARP:

- - - - - - - - - - - - - - - - - - - - Frame 6 - ARP - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

5 Cisc14F2C441 Broadcast 60 0:02:05.798 0.011.763 05/31/2001 06:53:04 AM ARP: R PA=[192.168.1.2]

HA=Cisc14F2C441 PRO=IP

DLC: ----- DLC Header -----

DLC:

DLC: Frame 128 arrived at 06:53:04.2439; frame size is 60 (003C hex) bytes.

DLC: Destination = BROADCAST FFFFFFFFFFFF, Broadcast

DLC: Source = Station Cisc14F2C441

DLC: Ethertype = 0806 (ARP)

DLC:

ARP: ----- ARP/RARP frame -----

ARP:

ARP: Hardware type = 1 (10Mb Ethernet)

ARP: Protocol type = 0800 (IP)

ARP: Length of hardware address = 6 bytes

ARP: Length of protocol address = 4 bytes

ARP: Opcode 2 (ARP reply)

ARP: Sender's hardware address = 00E01EF2C441

ARP: Sender's protocol address = [192.168.1.2]

ARP: Target hardware address = FFFFFFFFFFFF

ARP: Target protocol address = [192.168.1.2]

ARP:

ARP: 18 bytes frame padding

ARP:

Sniffer-A Trace

- - - - - - - - - - - - - - - - - - - - Frame 1 - DHCPDISCOVER - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

118 [192.168.1.1] [192.168.2.2] 618 0:00:51.212 0.489.912 05/31/2001 07:02:54 AM DHCP: Request,

Message type: DHCP Discover

DLC: ----- DLC Header -----

DLC:

DLC: Frame 118 arrived at 07:02:54.7463; frame size is 618 (026A hex) bytes.

DLC: Destination = Station 0005DC0BF2F4

DLC: Source = Station 003094248F72

DLC: Ethertype = 0800 (IP)

DLC:

IP: ----- IP Header -----

IP:

IP: Version = 4, header length = 20 bytes

IP: Type of service = 00

IP: 000. .... = routine

IP: ...0 .... = normal delay

IP: .... 0... = normal throughput

IP: .... .0.. = normal reliability

IP: .... ..0. = ECT bit - transport protocol will ignore the CE bit

IP: .... ...0 = CE bit - no congestion

IP: Total length = 604 bytes

IP: Identification = 52

IP: Flags = 0X

IP: .0.. .... = may fragment

IP: ..0. .... = last fragment

IP: Fragment offset = 0 bytes

IP: Time to live = 255 seconds/hops

IP: Protocol = 17 (UDP)

IP: Header checksum = 3509 (correct)

IP: Source address = [192.168.1.1]

IP: Destination address = [192.168.2.2]

IP: No options

IP:

UDP: ----- UDP Header -----

UDP:

UDP: Source port = 67 (BootPs/DHCP)

UDP: Destination port = 67 (BootPs/DHCP)

UDP: Length = 584

UDP: Checksum = 0A19 (correct)

UDP: [576 byte(s) of data]

UDP:

DHCP: ----- DHCP Header -----

DHCP:

DHCP: Boot record type = 1 (Request)

DHCP: Hardware address type = 1 (10Mb Ethernet)

DHCP: Hardware address length = 6 bytes

DHCP:

DHCP: Hops = 1

DHCP: Transaction id = 000005F4

DHCP: Elapsed boot time = 0 seconds

DHCP: Flags = 8000

DHCP: 1... .... .... .... = Broadcast IP datagrams

DHCP: Client self-assigned IP address = [0.0.0.0]

DHCP: Client IP address = [0.0.0.0]

DHCP: Next Server to use in bootstrap = [0.0.0.0]

DHCP: Relay Agent = [192.168.1.1]

DHCP: Client hardware address = 0005DCF2C441

DHCP:

DHCP: Host name = ""

DHCP: Boot file name = ""

DHCP:

DHCP: Vendor Information tag = 63825363

DHCP: Message Type = 1 (DHCP Discover)

DHCP: Maximum message size = 1152

DHCP: Client identifier = 00636973636F2D303065302E316566322E633434312D4574302F30

DHCP: Parameter Request List: 7 entries

DHCP: 1 = Client's subnet mask

DHCP: 6 = Domain name server

DHCP: 15 = Domain name

DHCP: 44 = NetBIOS over TCP/IP name server

DHCP: 3 = Routers on the client's subnet

DHCP: 33 = Static route

DHCP: 150 = Unknown Option

DHCP: Class identifier = 646F63736973312E30

DHCP: Option overload =3 (File and Sname fields hold options)

DHCP:

- - - - - - - - - - - - - - - - - - - - Frame 2 - DHCPOFFER - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

2 [192.168.2.2] [192.168.1.1] 347 0:00:51.214 0.002.133 05/31/2001 07:02:54 AM DHCP: Request,

Message type: DHCP Offer

DLC: ----- DLC Header -----

DLC:

DLC: Frame 119 arrived at 07:02:54.7485; frame size is 347 (015B hex) bytes.

DLC: Destination = Station 003094248F72

DLC: Source = Station 0005DC0BF2F4

DLC: Ethertype = 0800 (IP)

DLC:

IP: ----- IP Header -----

IP:

IP: Version = 4, header length = 20 bytes

IP: Type of service = 00

IP: 000. .... = routine

IP: ...0 .... = normal delay

IP: .... 0... = normal throughput

IP: .... .0.. = normal reliability

IP: .... ..0. = ECT bit - transport protocol will ignore the CE bit

IP: .... ...0 = CE bit - no congestion

IP: Total length = 333 bytes

IP: Identification = 41

IP: Flags = 0X

IP: .0.. .... = may fragment

IP: ..0. .... = last fragment

IP: Fragment offset = 0 bytes

IP: Time to live = 255 seconds/hops

IP: Protocol = 17 (UDP)

IP: Header checksum = 3623 (correct)

IP: Source address = [192.168.2.2]

IP: Destination address = [192.168.1.1]

IP: No options

IP:

UDP: ----- UDP Header -----

UDP:

UDP: Source port = 67 (BootPs/DHCP)

UDP: Destination port = 67 (BootPs/DHCP)

UDP: Length = 313

UDP: Checksum = A1F8 (correct)

UDP: [305 byte(s) of data]

UDP:

DHCP: ----- DHCP Header -----

DHCP:

DHCP: Boot record type = 2 (Request)

DHCP: Hardware address type = 1 (10Mb Ethernet)

DHCP: Hardware address length = 6 bytes

DHCP:

DHCP: Hops = 0

DHCP: Transaction id = 000005F4

DHCP: Elapsed boot time = 0 seconds

DHCP: Flags = 8000

DHCP: 1... .... .... .... = Broadcast IP datagrams

DHCP: Client self-assigned IP address = [0.0.0.0]

DHCP: Client IP address = [192.168.1.2]

DHCP: Next Server to use in bootstrap = [0.0.0.0]

DHCP: Relay Agent = [192.168.1.1]

DHCP: Client hardware address = 0005DCF2C441

DHCP:

DHCP: Host name = ""

DHCP: Boot file name = ""

DHCP:

DHCP: Vendor Information tag = 63825363

DHCP: Message Type = 2 (DHCP Offer)

DHCP: Server IP address = [192.168.2.2]

DHCP: Request IP address lease time = 172571 (seconds)

DHCP: Address Renewal interval = 86285 (seconds)

DHCP: Address Rebinding interval = 150999 (seconds)

DHCP: Subnet mask = [255.255.255.0]

DHCP: Domain Name Server address = [192.168.10.1]

DHCP: Domain Name Server address = [192.168.10.2]

DHCP: NetBIOS Server address = [192.168.10.1]

DHCP: NetBIOS Server address = [192.168.10.3]

DHCP: Domain name = "cisco.com"

DHCP:

- - - - - - - - - - - - - - - - - - - - Frame 3 - DHCPREQUEST - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

3 [192.168.1.1] [192.168.2.2] 618 0:00:51.240 0.025.974 05/31/2001 07:02:54 AM DHCP: Request,

Message type: DHCP Request

DLC: ----- DLC Header -----

DLC:

DLC: Frame 120 arrived at 07:02:54.7745; frame size is 618 (026A hex) bytes.

DLC: Destination = Station 0005DC0BF2F4

DLC: Source = Station 003094248F72

DLC: Ethertype = 0800 (IP)

DLC:

IP: ----- IP Header -----

IP:

IP: Version = 4, header length = 20 bytes

IP: Type of service = 00

IP: 000. .... = routine

IP: ...0 .... = normal delay

IP: .... 0... = normal throughput

IP: .... .0.. = normal reliability

IP: .... ..0. = ECT bit - transport protocol will ignore the CE bit

IP: .... ...0 = CE bit - no congestion

IP: Total length = 604 bytes

IP: Identification = 54

IP: Flags = 0X

IP: .0.. .... = may fragment

IP: ..0. .... = last fragment

IP: Fragment offset = 0 bytes

IP: Time to live = 255 seconds/hops

IP: Protocol = 17 (UDP)

IP: Header checksum = 3507 (correct)

IP: Source address = [192.168.1.1]

IP: Destination address = [192.168.2.2]

IP: No options

IP:

UDP: ----- UDP Header -----

UDP:

UDP: Source port = 67 (BootPs/DHCP)

UDP: Destination port = 67 (BootPs/DHCP)

UDP: Length = 584

UDP: Checksum = 4699 (correct)

UDP: [576 byte(s) of data]

UDP:

DHCP: ----- DHCP Header -----

DHCP:

DHCP: Boot record type = 1 (Request)

DHCP: Hardware address type = 1 (10Mb Ethernet)

DHCP: Hardware address length = 6 bytes

DHCP:

DHCP: Hops = 1

DHCP: Transaction id = 000005F4

DHCP: Elapsed boot time = 0 seconds

DHCP: Flags = 8000

DHCP: 1... .... .... .... = Broadcast IP datagrams

DHCP: Client self-assigned IP address = [0.0.0.0]

DHCP: Client IP address = [0.0.0.0]

DHCP: Next Server to use in bootstrap = [0.0.0.0]

DHCP: Relay Agent = [192.168.1.1]

DHCP: Client hardware address = 0005DCF2C441

DHCP:

DHCP: Host name = ""

DHCP: Boot file name = ""

DHCP:

DHCP: Vendor Information tag = 63825363

DHCP: Message Type = 3 (DHCP Request)

DHCP: Maximum message size = 1152

DHCP: Client identifier = 00636973636F2D303065302E316566322E633434312D4574302F30

DHCP: Server IP address = [192.168.2.2]

DHCP: Request specific IP address = [192.168.1.2]

DHCP: Request IP address lease time = 172571 (seconds)

DHCP: Parameter Request List: 7 entries

DHCP: 1 = Client's subnet mask

DHCP: 6 = Domain name server

DHCP: 15 = Domain name

DHCP: 44 = NetBIOS over TCP/IP name server

DHCP: 3 = Routers on the client's subnet

DHCP: 33 = Static route

DHCP: 150 = Unknown Option

DHCP: Class identifier = 646F63736973312E30

DHCP: Option overload =3 (File and Sname fields hold options)

DHCP:

- - - - - - - - - - - - - - - - - - - - Frame 4 - DHCPACK - - - - - - - - - - - - - - - - - - - -

Frame Status Source Address Dest. Address Size Rel. Time Delta Time Abs. Time Summary

4 [192.168.2.2] [192.168.1.1] 347 0:00:51.240 0.000.153 05/31/2001 07:02:54 AM DHCP: Request,

Message type: DHCP Ack

DLC: ----- DLC Header -----

DLC:

DLC: Frame 121 arrived at 07:02:54.7746; frame size is 347 (015B hex) bytes.

DLC: Destination = Station 003094248F72

DLC: Source = Station 0005DC0BF2F4

DLC: Ethertype = 0800 (IP)

DLC:

IP: ----- IP Header -----

IP:

IP: Version = 4, header length = 20 bytes

IP: Type of service = 00

IP: 000. .... = routine

IP: ...0 .... = normal delay

IP: .... 0... = normal throughput

IP: .... .0.. = normal reliability

IP: .... ..0. = ECT bit - transport protocol will ignore the CE bit

IP: .... ...0 = CE bit - no congestion

IP: Total length = 333 bytes

IP: Identification = 42

IP: Flags = 0X

IP: .0.. .... = may fragment

IP: ..0. .... = last fragment

IP: Fragment offset = 0 bytes

IP: Time to live = 255 seconds/hops

IP: Protocol = 17 (UDP)

IP: Header checksum = 3622 (correct)

IP: Source address = [192.168.2.2]

IP: Destination address = [192.168.1.1]

IP: No options

IP:

UDP: ----- UDP Header -----

UDP:

UDP: Source port = 67 (BootPs/DHCP)

UDP: Destination port = 67 (BootPs/DHCP)

UDP: Length = 313

UDP: Checksum = 7DF6 (correct)

UDP: [305 byte(s) of data]

UDP:

DHCP: ----- DHCP Header -----

DHCP:

DHCP: Boot record type = 2 (Request)

DHCP: Hardware address type = 1 (10Mb Ethernet)

DHCP: Hardware address length = 6 bytes

DHCP:

DHCP: Hops = 0

DHCP: Transaction id = 000005F4

DHCP: Elapsed boot time = 0 seconds

DHCP: Flags = 8000

DHCP: 1... .... .... .... = Broadcast IP datagrams

DHCP: Client self-assigned IP address = [0.0.0.0]

DHCP: Client IP address = [192.168.1.2]

DHCP: Next Server to use in bootstrap = [0.0.0.0]

DHCP: Relay Agent = [192.168.1.1]

DHCP: Client hardware address = 0005DCF2C441

DHCP:

DHCP: Host name = ""

DHCP: Boot file name = ""

DHCP:

DHCP: Vendor Information tag = 63825363

DHCP: Message Type = 5 (DHCP Ack)

DHCP: Server IP address = [192.168.2.2]

DHCP: Request IP address lease time = 172800 (seconds)

DHCP: Address Renewal interval = 86400 (seconds)

DHCP: Address Rebinding interval = 151200 (seconds)

DHCP: Subnet mask = [255.255.255.0]

DHCP: Domain Name Server address = [192.168.10.1]

DHCP: Domain Name Server address = [192.168.10.2]

DHCP: NetBIOS Server address = [192.168.10.1]

DHCP: NetBIOS Server address = [192.168.10.3]

DHCP: Domain name = "cisco.com"

DHCP:

Troubleshoot DHCP When Client Workstations are Unable to Obtain DHCP Addresses

Case Study #1: DHCP Server on Same LAN Segment or VLAN as DHCP Client

When the DHCP server and client reside on the same LAN segment or VLAN and the client is unable to obtain an IP address from a DHCP server. But it is unlikely that the local router causes a DHCP problem. The problem is related to the devices that connect the DHCP server and DHCP client. However, the problem can be with the DHCP server or client itself. These modules help troubleshoot and determine which device causes an issue.

Note: To configure the DHCP server on a per vlan basis, define different DHCP pools for every VLAN that serves DHCP addresses to your clients.

Case Study #2: DHCP Server and DHCP Client are Separated by a Router Configured for DHCP/BootP Relay Agent Functionality

When the DHCP server and client reside on the different LAN segments or VLANs, the router functions as a DHCP/BootP Relay Agent that is responsible for forwarding the DHCPREQUEST to the DHCP server. Additional steps are required to troubleshoot the DHCP/BootP Relay Agent, as well as the DHCP server and client. If you track these modules, you can determine which device causes the issues.

DHCP Server on Router Fails to Assign Adresses with a POOL EXHAUSTED Error

It is possible that some addresses are still held by clients, even if they are released from the pool. This can be verified by the show ip dhcp conflict  output. An address conflict occurs when two hosts use the same IP address. At the address assignment, the DHCP checks for conflicts with ping and gratuitous ARP.

If a conflict is detected, the address is removed from the pool. The address is assigned until the administrator resolves the conflict. Configure no ip dhcp conflict logging  to resolve this issue.

DHCP Troubleshoot Modules

Understand Where DHCP Problems Can Occur

DHCP problems can arise due to a multitude of reasons. The most common reasons are configuration issues. However, many DHCP problems can be caused by software defects in systems, Network Interface Card (NIC) drivers, or DHCP/BootP Relay Agents that run on routers. Due to the number of potentially problematic areas, a systematic approach to troubleshoot is required.

Short List of Possible Causes of DHCP Problems:

- Catalyst switch default configuration

- DHCP/BootP Relay Agent configuration

- NIC compatibility issue or DHCP feature issue

- Faulty NIC or improper NIC driver installation

- Intermittent network outages due to frequent spanning tree computations

- Operating system behavior or software defect

- DHCP server scope configuration or software defect

- Cisco Catalyst switch or Cisco IOS DHCP/BootP Relay Agent software defect

- Unicast Reverse Path Forwarding (uRPF) check failing because the DHCP offer is received on a different interface than expected. When the Reverse Path Forwarding (RPF) feature is enabled on an interface, a Cisco router can drop Dynamic Host Configuration Protocol (DHCP) and BOOTstrap Protocol (BOOTP) packets that have source addresses of 0.0.0.0 and destination addresses of 255.255.255.255. The router can also drop all IP packets that have a multicast IP destination at the interface. This issue is documented in Cisco bug ID CSCdw31925

Note Only registeredCisco clinents can access bug reports.

- DHCP database agent is not used, but DHCP conflict logging isnotdisabled

A. Verify Physical Connectivity

This procedure is applicable to all case studies.

First, verify physical connectivity of a DHCP client and server. If connected to a Catalyst switch, verify that both the DHCP client and server have physical connectivity. For Cisco IOS based switches such as the Catalyst 2900XL/3500XL/2950/3550, the equivalent command to show port status  is show interface <interface>.  If the state of the interface is anything other than <interface> is up, line protocol is up, the port does not pass traffic, not even DHCP client requests. The output from the commands:

Switch#show interface fastEthernet 0/1

FastEthernet0/1 is up, line protocol is up

Hardware is Fast Ethernet, address is 0030.94dc.acc1 (bia 0030.94dc.acc1)

If the physical connection has been verified and there is indeed no link between the Catalyst switch and DHCP client use the Troubleshooting Cisco Catalyst Switches to NIC Compatibility Issuessection to troubleshoot issues in regard to the physical layer connectivity issue.

Excessive data link errors cause ports on some Catalyst switches to go into anerrdisabledstate. For more information refer to Errdisable Port State Recovery on the Cisco IOS Platforms, which describe the errdisable state, explain how to recover from it, and provide examples of recovery from this state.

B. Configure the Client Workstation and Static IP to Test Network Connectivity

This procedure is applicable to all case studies.

When you troubleshoot any DHCP Issue, it is important to configure a static IP address on a client workstation in order to verify network connectivity. If the workstation is unable to reach network resources despite the fact that it has a statically configured IP address, the root cause of the problem is not DHCP. At this point, you are required to troubleshoot the network connectivity.

C. Verify Issue as a Startup Problem

This procedure is applicable to all case studies.

If the DHCP client is unable to obtain an IP address from the DHCP server on startup, you can manually force the client to send a DHCP request. Issue the the next steps to manually obtain an IP address from a DHCP server for the listed OS.

Microsoft Windows 95/98/ME:

1. Click theStartbutton and run the WINIPCFG.exe program.

2. Click theRelease Allbutton, tracked by theRenew Allbutton.

3. Is the DHCP client now able to obtain an IP address?

IP Configuration Window

Microsoft Windows NT/2000:

1. Enter  cmd  in the Start/Run  field to open a command prompt window.

2. Issue the command ipconfig/renew  in the command prompt window.

3. Is the DHCP client now able to obtain an IP Address?

Command Line Prompt

If the DHCP client is able to obtain an IP address with a manual renewal of the IP address after the PC has completed the bootup process, the issue is most likely a DHCP startup issue. If the DHCP client is attached to a Cisco Catalyst switch, the problem is most likely due to a configuration issue that deals with STP portfast and/or channeling and trunking. Other possibilities include NIC card issues and switch port startup issues. ReviewSteps D andE to rule out switch port configuration and NIC card issues as the root cause of the DHCP problem.

D. Verify Switch Port Configuration (STP Portfast and Other Commands)

If the switch is a Catalyst 2900/4000/5000/6000, verify that the port has STP portfast enabled and trunking/channeling disabled. The default configuration is STP portfast disabled and trunking/channeling auto, if applicable. For the 2900XL/3500XL/2950/3550 switches, STP portfast is the only required configuration. These configuration changes resolve the most common DHCP client issues that occur with an initial installation of a Catalyst switch.

For more documentation about the necessary switch port configuration requirements for DHCP to operate properly when connected to Catalyst switches, refer to  Using Portfast and Other Commands to Fix Workstation Startup Connectivity Delays.

After you have reviewed that document, you can continue to troubleshoot these issues.

E. Check for Known NIC Card or Catalyst Switch Issues

If the Catalyst switch configuration is correct, it is possible that a software compatibility issue can exist on the Catalyst switch or DHCP client NIC that could cause the DHCP issues. The next step to troubleshoot is to review Troubleshooting Cisco Catalyst Switches to NIC Compatibility Issues and rule out any software issues with the Catalyst switch or NIC that contribute to the problem.

Knowledge of the DHCP client OS as well as specific NIC information such as the manufacturer, model, and driver version is needed to properly rule out any compatibility issues.

F. Distinguishwhether DHCP Clients Obtain IP Address on the Same Subnet or VLAN as DHCP Server

It is important to distinguish whether or not DHCP is functions correctly when the client is on same subnet or VLAN as the DHCP server. If the DHCP does work correctly on the same subnet or VLAN as the DHCP server, the DHCP issue is mostly caused by the DHCP/BootP Relay Agent. If the problem persists even when you test DHCP on the same subnet or VLAN as the DHCP server, the problem can actually be with the DHCP server.

G. Verify Router DHCP/BootP Relay Configuration

To verify the configuration:

1. When you configure the DHCP relay on a router, verify that the ip helper-address  command is located on the correct interface. The ip helper-address  command must be present on the inbound interface of the DHCP client workstations and must be directed to the correct DHCP server.

2. Verify that the global configuration command no service dhcp  is not present. This configuration parameter disables all DHCP server and relay functionality on the router. The default configuration,  service dhcp, does not appear in the configuration, and is the default configuration command. If th eservice dhcp   is not enabled, the clients do not receive the IP addresses from the DHCP server.

Note: In routers that run older Cisco IOS releases, the   ip bootp server  command handles the DHCP relay agent function instead of the   service dhcp  command. Because of this, the   ip bootp server   command needs to be enabled in these routers if the   ip helper-address  command is configured to forward DHCP UDP broadcasts and properly act as a DHCP relay agent on behalf of the DHCP client.

3. When you use  ip helper-address  commands to forward UDP broadcasts to a subnet broadcast address, verify that  no ip directed-broadcast  is not configured on any outbound interface that the UDP broadcast packets needs to traverse. The  no ip directed-broadcast   blocks on any translation of a directed broadcast to physical broadcasts. This interface configuration is the default configuration in software versions 12.0 and higher.

4. When DHCP broadcasts are forwarded to the DHCP server subnet broadcast address a software issue can arise. When you troubleshoot DHCP issues, attempt to forward DHCP UDP broadcasts to the DHCP server IP address:

version 12.0

service timestamps debug uptime

service timestamps log uptime

no service password-encryption

no service dhcp

!--- This configuration command will disable all DHCP server and relay functionality on the router.

hostname router

!

!

!

interface Ethernet0

ip address 192.168.2.1 255.255.255.0

no ip directed-broadcast

!--- This configuration will prevent translation of a directed broadcast to a physical broadcast.

interface Ethernet1

!--- DHCP client workstations reside of this interface.

ip address 192.168.1.1 255.255.255.0

ip helper-address 192.168.2.255

!--- IP helper-address pointing to DHCP server's subnet.

no ip directed-broadcast

!

!

!

line con 0

exec-timeout 0 0

transport input none

line aux 0

line vty 0 4

login

!

end

H. Subscriber Identification (82) Option Turned On

The DHCP relay agent information (option 82) feature enables the DHCP relay agents (Catalyst switches) to include information about itself and the attached client when it forwards DHCP requests from a DHCP client to a DHCP server.

The DHCP server can use this information to assign IP addresses, perform access control, and set quality of service (QoS) and security policies (or other parameter-assignment policies) for each subscriber of a service-provider network. When DHCP snooping is enabled on a switch, it automatically enables option 82. If the DHCP server is not configured to handle the packets with option 82, it ceases to allocate the address to that request. In order to resolve this issue, disable the subscriber identification option (82) in the switches (relay agents) with the global configuration command,no ip dhcp relay information option.

I. DHCP Database Agent and DHCP Conflict Logging

A DHCP database agent is any host—for example, an FTP, TFTP, or RCP server—that stores the DHCP bindings database. You can configure multiple DHCP database agents, and you can configure the interval between database updates and transfers for each agent. Use the ip dhcp database command to configure a database agent and database agent parameters.

If you choose not to configure a DHCP database agent, disable the recording of DHCP address conflicts on the DHCP server. Execute the noip dhcp conflict logging command to disable the DHCP address conflict logging. Clear the previously logged conflicts with clear ip dhcp conflict.

If this fails to disable the conflict logging, this error message appears:

%DHCPD-4-DECLINE_CONFLICT: DHCP address conflict: client

J. Check CDP for IP Phone Connections

When the switchport that is connected to the Cisco IP phone has Cisco Discovery Protocol (CDP) disabled, the DHCP server cannot assign an appropriate IP address to the phone. The DHCP server tends to assign the IP address that belongs to the data VLAN / subnet of the switchport. If the CDP is enabled, the switch is able to detect that the Cisco IP Phone requests the DHCP and can provide the correct subnet information. The DHCP server then is able to allot an IP address from the voice VLAN / subnet pool. There are no explicit steps required to bind the dhcp service to the voice vlan.

K. Remove Down SVI Disrupts DHCP Snooping Operation

On the Cisco Catalyst 6500 series switches, an SVI (in shutdown state) is created automatically after it configures the DHCP to snoop for a particular VLAN. The presence of this SVI has direct implications on the correct operation of DHCP snooping.

DHCP snooping on the Cisco Catalyst 6500 series switches that run Native Cisco IOS is implemented mostly on Route Processor (RP or MSFC), not on Switch Processor (SP or Supervisor). The Cisco Catalyst 6500 series intercepts packets in hardware with VACLs that supply the packets to a Local Target Logic (LTL) subscribed to by the RP. Once the frames enter the RP, they first need to be associated with an L3 Interface (SVI) IDB before they can be passed off to the snooping part. Without an SVI, this IDB does not exist, and the packets get dropped in the RP.

L. Limited Broadcast Address

When a DHCP client sets the broadcast bit in a DHCP packet, the DHCP server and relay agent send DHCP messages to clients with the all-ones broadcast address (255.255.255.255). If the ip broadcast-address command has been configured to send a network broadcast, the all-ones broadcast sent by DHCP is overridden. In order to remedy this situation, use the ip dhcp limited-broadcast-address command to ensure that a configured network broadcast does not override the default DHCP behavior.

Some DHCP clients can only accept an all-ones broadcast and are not able to acquire a DHCP address unless this command is configured on the router interface connected to the client.

M. Debug DHCP With Router Debug Commands

Verify Router Receives DHCP Request with debug Commands

On routers that support software that processes DHCP packets, you can verify whether a router receives the DHCP request from the client. The DHCP process fails if the router does not receive requests from the client. In this step, configure an access-list to debug output. This access-list is used only to debug a command and is not intrusive to the router.

In global configuration mode, enter this access-list:

access-list 100 permit ip host 0.0.0.0 host 255.255.255.255

In exec mode, enter this debug command:

debug ip packet detail 100

Sample Output

Router#debug ip packet detail 100

IP packet debugging is on (detailed) for access list 100

Router#

00:16:46: IP: s=0.0.0.0 (Ethernet4/0), d=255.255.255.255, len 604, rcvd 2

00:16:46: UDP src=68, dst=67

00:16:46: IP: s=0.0.0.0 (Ethernet4/0), d=255.255.255.255, len 604, rcvd 2

00:16:46: UDP src=68, dst=67

From this output example, it is clear that the router actively receives the DHCP requests from the client. This output only shows a summary of the packet and not the packet itself. Therefore, it is not possible to determine if the packet is correct. Nevertheless, the router did receive a broadcast packet with the source and destination IP and UDP ports that are correct for DHCP.

Verify Router Receives and Forwards DHCP Request with debug ip udp Command

The debug ip udp  command can trace the path of a DHCP request through a router. However, this debug is intrusive in a production environment, since all processed switched UDP packets are displayed to the console. This debug command must not be used in production.

Warning: The   debug ip udp  command is intrusive, and can cause high Central Processing Unit (CPU) utilization.

In exec mode, enter this debug command:  debug ip udp

Sample Output

Router#debug ip udp

UDP packet debugging is on

Router#

00:18:48: UDP: rcvd src=0.0.0.0(68), dst=255.255.255.255(67), length=584

!--- Router receiving DHCPDISCOVER from DHCP client.

00:18:48: UDP: sent src=192.168.1.1(67), dst=192.168.2.2(67), length=604

!--- Router forwarding DHCPDISCOVER unicast to DHCP server using DHCP/BootP Relay Agent source IP address.

00:18:48: UDP: rcvd src=192.168.2.2(67), dst=192.168.1.1(67), length=313

!--- Router receiving DHCPOFFER from DHCP server directed to DHCP/BootP Relay Agent IP address.

00:18:48: UDP: sent src=0.0.0.0(67), dst=255.255.255.255(68), length=333

!--- Router forwarding DHCPOFFER from DHCP server to DHCP client via DHCP/BootP Relay Agent.

00:18:48: UDP: rcvd src=0.0.0.0(68), dst=255.255.255.255(67), length=584

!--- Router receiving DHCPREQUEST from DHCP client.

00:18:48: UDP: sent src=192.168.1.1(67), dst=192.168.2.2(67), length=604

!--- Router forwarding DHCPDISCOVER unicast to DHCP server using DHCP/BootP Relay Agent source IP address.

00:18:48: UDP: rcvd src=192.168.2.2(67), dst=192.168.1.1(67), length=313

!--- Router receiving DHCPACK (or DHCPNAK) from DHCP directed to DHCP/BootP Relay Agent IP address.

00:18:48: UDP: sent src=0.0.0.0(67), dst=255.255.255.255(68), length=333

!--- Router forwarding DHCPACK (or DHCPNAK) to DHCP client via DHCP/BootP Relay Agent.

00:18:48: UDP: rcvd src=192.168.1.2(520), dst=255.255.255.255(520), length=32

!--- DHCP client verifying IP address not in use by sending ARP request for its own IP address.

00:18:50: UDP: rcvd src=192.168.1.2(520), dst=255.255.255.255(520), length=32

!--- DHCP client verifying IP address not in use by sending ARP request for its own IP address.

Verify Router Receives and Forwards DHCP Request with debug ip dhcp server packet Command

If the router Cisco IOS is 12.0.x.T or 12.1 and supports the Cisco IOS DHCP server functionality, you can use the  debug ip dhcp server packet  command. This debug was intended for use with the Cisco IOS DHCP server feature and to troubleshoot the DHCP/BootP Relay Agent feature as well. As with the previous steps, router debugs do not provide an exact determination of the problem since the actual packet cannot be viewed. However, debugs do allow inferences to be made with regards to DHCP processing. In exec mode, enter this debug command:

debug ip dhcp server packet

Router#debug ip dhcp server packet

00:20:54: DHCPD: setting giaddr to 192.168.1.1.

!--- Router received DHCPDISCOVER/REQUEST/INRORM and setting Gateway IP address to 192.168.1.1 for forwarding.

00:20:54: DHCPD: BOOTREQUEST from 0063.6973.636f.2d30.3065.302e.3165.6632.2e63..

!--- BOOTREQUEST includes DHCPDISCOVER, DHCPREQUEST, and DHCPINFORM.

!--- 0063.6973.636f.2d30.3065.302e.3165.6632.2e63 indicates client identifier.

00:20:54: DHCPD: forwarding BOOTREPLY to client 00e0.1ef2.c441.

!--- BOOTREPLY includes DHCPOFFER and DHCPNAK.

!--- Client's MAC address is 00e0.1ef2.c441.

00:20:54: DHCPD: broadcasting BOOTREPLY to client 00e0.1ef2.c441.

!--- Router is forwarding DHCPOFFER or DHCPNAK broadcast on local LAN interface.

00:20:54: DHCPD: setting giaddr to 192.168.1.1.

!--- Router received DHCPDISCOVER/REQUEST/INFORM and set Gateway IP address to 192.168.1.1 for forwarding.

00:20:54: DHCPD: BOOTREQUEST from 0063.6973.636f.2d30.3065.302e.3165.6632.2e63..

!--- BOOTREQUEST includes DHCPDISCOVER, DHCPREQUEST, and DHCPINFORM.

!--- 0063.6973.636f.2d30.3065.302e.3165.6632.2e63 indicates client identifier.

00:20:54: DHCPD: forwarding BOOTREPLY to client 00e0.1ef2.c441.

!--- BOOTREPLY includes DHCPOFFER and DHCPNAK.

!--- Client's MAC address is 00e0.1ef2.c441.

00:20:54: DHCPD: broadcasting BOOTREPLY to client 00e0.1ef2.c441.

!--- Router is forwarding DHCPOFFER or DHCPNAK broadcast on local LAN interface.

Run Multiple Debugs Simultaneously

When you run multiple debugs simultaneously, a fair amount of information can be discovered with regard to the operation of the DHCP/BootP Relay Agent and server. If you use the previous outlines to troubleshoot, you can make inferences about where the DHCP/BootP Relay Agent functionality does not operate correctly.

IP: s=0.0.0.0 (Ethernet0), d=255.255.255.255, len 604, rcvd 2

UDP src=68, dst=67

UDP: rcvd src=0.0.0.0(68), dst=255.255.255.255(67), length=584

DHCPD: setting giaddr to 192.168.1.1.

UDP: sent src=192.168.1.1(67), dst=192.168.2.2(67), length=604

IP: s=192.168.1.1 (local), d=192.168.2.2 (Ethernet1), len 604, sending

UDP src=67, dst=67

DHCPD: BOOTREQUEST from 0063.6973.636f.2d30.3030.302e.3030.3030.2e30.3030.312d.4574.30 forwarded to 192.168.2.2.

IP: s=192.168.2.2 (Ethernet1), d=192.168.1.1, len 328, rcvd 4

UDP src=67, dst=67

UDP: rcvd src=192.168.2.2(67), dst=192.168.1.1(67), length=308

DHCPD: forwarding BOOTREPLY to client 0000.0000.0001.

DHCPD: broadcasting BOOTREPLY to client 0000.0000.0001.

UDP: sent src=0.0.0.0(67), dst=255.255.255.255(68), length=328

IP: s=0.0.0.0 (Ethernet0), d=255.255.255.255, len 604, rcvd 2

UDP src=68, dst=67

UDP: rcvd src=0.0.0.0(68), dst=255.255.255.255(67), length=584

DHCPD: setting giaddr to 192.168.1.1.

UDP: sent src=192.168.1.1(67), dst=192.168.2.2(67), length=604

IP: s=192.168.1.1 (local), d=192.168.2.2 (Ethernet1), len 604, sending

UDP src=67, dst=67

DHCPD: BOOTREQUEST from 0063.6973.636f.2d30.3030.302e.3030.3030.2e30.3030.312d.4574.30 forwarded to 192.168.2.2.

IP: s=192.168.2.2 (Ethernet1), d=192.168.1.1, len 328, rcvd 4

UDP src=67, dst=67

UDP: rcvd src=192.168.2.2(67), dst=192.168.1.1(67), length=308

DHCPD: forwarding BOOTREPLY to client 0000.0000.0001.

DHCPD: broadcasting BOOTREPLY to client 0000.0000.0001.

UDP: sent src=0.0.0.0(67), dst=255.255.255.255(68), length=328.

Obtain Sniffer Trace and Determine Root Cause of DHCP Problem

Review the  Decode Sniffer Trace of DHCP Client and Server on Same LAN Segment  and  Decode Sniffer Trace of DHCP Client and Server Separated by Router Configured as a DHCP Relay Agent  sections

to decipher DHCP packet traces.

For information on how to obtain sniffer traces with the Switched Port Analyzer (SPAN) feature on Catalyst switches, refer to  Configuring the Catalyst Switched Port Analyzer (SPAN) Configuration Example.

Alternative Method of Packet Decode with Debug on Router

With the  debug ip packet detail dump <acl>  command on a Cisco router, it is possible to get an entire packet in hex displayed in the system log or Command Line Interface (CLI). Review the  Verify Router Receives DHCP Request with debug Commands  and Verify Router Receives DHCP Request and Forwards Request to DHCP Server with debug Commands  sections above, along with the dump keyword added to the access-list, to get the same debug information, but with the packet detail in hex. To determine the contents of the packet, the packet needs to be translated. An example is given in Appendix A.

Appendix A: Cisco IOS DHCP Sample Configuration

The DHCP server database is organized as a tree. The root of the tree is the address pool for natural networks, branches are subnetwork address pools, and leaves are manual bindings to clients. Subnetworks inherit network parameters and clients inherit subnetwork parameters. Therefore, common parameters, for example the domain name, must be configured at the highest (network or subnetwork) level of the tree.

For more information on how to configure DHCP and the commands associated with it, refer to the  DHCP Configuration Task List.

version 12.1

!

service timestamps debug uptime

service timestamps log uptime

no service password-encryption

!

hostname Router

!

enable password cisco

ip subnet-zero

no ip domain-lookup

ip dhcp excluded-address 10.10.1.1 10.10.1.199

!--- Address range excluded from DHCP pools.

ip dhcp pool test_dhcp

!--- DHCP pool (scope) name is test_dhcp.

network 10.10.1.0 255.255.255.0

!--- DHCP pool (address will be assigned in this range) for associated Gateway IP address.

default-router 10.10.1.1

!--- DHCP option for default gateway.

dns-server 10.30.1.1

!--- DHCP option for DNS server(s).

netbios-name-server 10.40.1.1

!--- DHCP option for NetBIOS name server(s) (WINS).

lease 0 0 1

!--- Lease time.

interface Ethernet0

description DHCP Client Network

ip address 10.10.1.1 255.255.255.0

no ip directed-broadcast

!

interface Ethernet1

description Server Network

ip address 10.10.2.1 255.255.255.0

no ip directed-broadcast

!

line con 0

transport input none

line aux 0

transport input all

line vty 0 4

login

!

end

Note: Notice that the command  subnet prefix-length  is not essential for the normal operation of the DHCP pool. The use of this command is focused on scenarios where subnet allocation pools are needed, for more information on this command refer to the section  Subnet Allocation Server Operation  from the document  Configuring the DHCP Server On-Demand Address Pool Manager.

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