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Providing Connectivity Using ATM Routed Bridge Encapsulation over PVCs
The Providing Connectivity Using ATM Routed Bridge Encapsulation over PVCs feature provides the functionality of bridged
ATM interface support to ATM switched virtual circuits (SVCs). Unlike permanent virtual circuits (PVCs), SVCs must be triggered
by ongoing traffic and can be brought down when idle for some time. The SVCs are triggered, if down, and the traffic is passed
on to the SVCs belonging to bridged ATM interface.
ATM routed bridge encapsulation (RBE) is used to route IP over bridged RFC 1483 Ethernet traffic from a stub-bridged LAN.
Prerequisites for Providing Connectivity Using ATM Routed Bridge Encapsulation over PVCs
When ATM SVCs are used, support for a form of bridging, such as integrated routing and bridging, is required.
Before configuring connectivity from a remote bridged Ethernet network to a routed network using ATM routed bridge encapsulation,
you must understand the concepts in the Understanding Broadband Access Aggregation module.
Restrictions for Providing Connectivity Using ATM Routed Bridge Encapsulation over PVCs
Unlike PVCs, SVCs must be triggered by ongoing traffic and might be brought down after they have been idle for some time.
The Bridged 1483 Encapsulated Traffic over ATM SVCs feature allows for the SVC to be triggered if down, and to pass the traffic
on to the SVCs belonging to the bridged ATM interface.
ATM RBE does not support MAC-layer access lists; only IP access lists are supported.
Information About Providing Connectivity Using ATM Routed Bridge Encapsulation over PVCs
Overview on Bridged 1483 Encapsulated Traffic over ATM SVCs
ATM RBE is used to route IP over bridged RFC 1483 Ethernet traffic from
a stub-bridged LAN.
The figure below shows an ATM subinterface on a headend router that is
configured to function in ATM routed-bridge encapsulation mode. This
configuration is useful when a remote bridged Ethernet network device needs
connectivity to a routed network via a device bridging from an Ethernet LAN to
an ATM RFC 1483 bridged encapsulation.
Figure 1. ATM Routed Bridge Encapsulation
Because PVCs are statically configured along the entire path between
the end systems, it would not be suitable to route bridged encapsulated traffic
over them when the user wants to configure the virtual circuits (VCs)
dynamically and tear down the VCs when there is no traffic.
ATM RBE Subinterface Grouping by PVC Range
You can configure ATM routed bridge encapsulation using an ATM PVC range rather than individual PVCs. When you configure a
PVC range for routed bridge encapsulation, a point-to-point subinterface is created for each PVC in the range. The number
of PVCs in a range can be calculated using the following formula:
number of PVCs = (end-vpi - start-vpi + 1) x (end-vci - start-vci +1)
Subinterface numbering begins with the subinterface on which the PVC range is configured and increases sequentially through
the range.
Note
You cannot explicitly configure the individual point-to-point subinterfaces created by the PVC range on a point-to-point subinterface.
All the point-to-point subinterfaces in the range share the same configuration as the subinterface on which the PVC range
is configured.
DHCP Option 82 Support for RBE
The DHCP relay agent information option (option 82) enables a Dynamic Host Configuration Protocol (DHCP) relay agent to include
information about itself when forwarding client-originated DHCP packets to a DHCP server. The DHCP server can use this information
to implement IP address or other parameter-assignment policies.
The DHCP Option 82 Support for RBE feature provides support for the DHCP relay agent information option when ATM RBE is used.
The figure below shows a typical network topology in which ATM RBE and DHCP are used. The aggregation router that is using
ATM RBE is also serving as the DHCP relay agent.
Figure 2. Network Topology Using ATM RBE and DHCP
This feature communicates information to the DHCP server using a suboption of the DHCP relay agent information option called
agent remote ID . The information sent in the agent remote ID includes an IP address identifying the relay agent and information about the
ATM interface and the PVC over which the DHCP request came in. The DHCP server can use this information to make IP address
assignments and security policy decisions.
The figure below shows the format of the agent remote ID suboption.
Figure 3. Format of the Agent Remote ID Suboption
The table below describes the agent remote ID suboption fields displayed in the figure above.
Table 1. Agent Remote ID Suboption Field Descriptions
Field
Description
Port Type
Port type. The value 0x01 indicates RBE. (1 byte)
Version
Option 82 version. The value 0x01 specifies the RBE version of Option 82 (1 byte).
Reserved
RBE reserved (2 bytes).
NAS IP Address
One of the interfaces on the DHCP relay agent. The
rbenasip command can be used to specify which IP address will be used. (4 bytes)
NAS Port
RBE-enabled virtual circuit on which the DHCP request has come in. See the figure below for the format of this field. (4
bytes)
The figure below shows the format of the network access server (NAS) port field in the agent remote ID suboption.
Figure 4. Format of the NAS Port Field
The figure below shows the format of the interface field. If there is no module, the value of the module bit is 0.
Figure 5. Format of the Interface Field
DHCP Lease Limit per ATM RBE Unnumbered Interface
The DHCP lease limit per ATM RBE Unnumbered Interface feature is enabled on a Cisco IOS DHCP relay agent connected to clients
through unnumbered interfaces. The relay agent keeps information about the DHCP leases offered to the clients per subinterface.
When a DHCPACK message is forwarded to the client, the relay agent increments the number of leases offered to clients on that
subinterface. If a new DHCP client tries to obtain an IP address and the number of leases has already reached the configured
lease limit, DHCP messages from the client will be dropped and will not be forwarded to the DHCP server.
If this feature is enabled on the Cisco IOS DHCP server directly connected to clients through unnumbered interfaces, the server
allocates addresses and increments the number of leases per subinterface. If a new client tries to obtain an IP address, the
server will not offer an IP address if the number of leases on the subinterface has already reached the configured lease limit.
Benefits of Providing Connectivity Using ATM Routed Bridge Encapsulation
Bridged IP packets received on an ATM interface configured in routed-bridge mode are routed via the IP header. Such interfaces
take advantage of the characteristics of a stub LAN topology commonly used for digital subscriber line (DSL) access and offer
increased performance and flexibility over integrated routing and bridging (IRB).
Another benefit of ATM RBE is that it reduces the security risk associated with normal bridging or IRB by reducing the size
of the nonsecured network. By using a single VC allocated to a subnet (which could be as small as a single IP address), ATM
RBE uses an IP address in the subnet to limit the "trust environment" to the premises of a single customer.
ATM RBE supports Cisco Express Forwarding (CEF), fast switching, and process switching.
The DHCP Option 82 Support for RBE feature enables those service providers to use DHCP to assign IP addresses and DHCP option
82 to implement security and IP address assignment policies.
The DHCP Lease Limit per ATM RBE Unnumbered Interface feature allows an Internet service provider (ISP) to globally limit
the number of leases available to clients per household or connection.
How to Configure ATM Routed Bridge Encapsulation over PVCs
Configuring ATM Routed Bridge Encapsulation Using PVCs
Perform the following task to configure ATM RBE using PVCs. Only the specified network layer (IP) is routed. Any remaining
protocols can be passed on to bridging or other protocols. In this manner, ATM RBE can be used to route IP, while other protocols
(such as IPX) are bridged normally.
To confirm that ATM RBE is enabled, use the
showarp command and the
showipcacheverbose command in privileged EXEC mode:
Router# show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 209.165.201.51 6 0001.c9f2.a81d ARPA Ethernet3/1
Internet 209.165.201.49 - 0060.0939.bb55 ARPA Ethernet3/1
Internet 209.165.202.128 30 0010.0ba6.2020 ARPA Ethernet3/0
Internet 209.165.201.52 6 00e0.1e8d.3f90 ARPA ATM1/0.4
Internet 209.165.201.53 5 0007.144f.5d20 ARPA ATM1/0.2
Internet 209.165.202.129 - 0060.0939.bb54 ARPA Ethernet3/0
Internet 209.165.201.125 30 00b0.c2e9.bc55 ARPA Ethernet3/1#
Router# show ip cache verbose
IP routing cache 3 entries, 572 bytes
9 adds, 6 invalidates, 0 refcounts
Minimum invalidation interval 2 seconds, maximum interval 5 seconds,
quiet interval 3 seconds, threshold 0 requests
Invalidation rate 0 in last second, 0 in last 3 seconds
Last full cache invalidation occurred 00:30:34 ago
Prefix/Length Age Interface Next Hop
209.165.201.51/32-24 00:30:10 Ethernet3/1 10.1.0.51 14 0001C9F2A81D00600939 BB550800
209.165.202.129/32-24 00:00:04 ATM1/0.2 10.8.100.50 28 00010000AAAA030080C2000700000007144F5D2000600939 BB1C0800
209.165.201.125/32-24 00:06:09 ATM1/0.4 10.8.101.35 28 00020000AAAA030080C20007000000E01E8D3F9000600939 BB1C0800
Configuring DHCP Option 82 for RBE
Perform this task to configure the DHCP Option 82 Support for RBE feature.
Before you begin
DHCP option 82 support must be configured on the DHCP relay agent using the ipdhcprelayinformationoption command before you can use the DHCP Option 82 Support for RBE feature.
SUMMARY STEPS
enable
configureterminal
ipdhcprelayinformationoption
rbenasipsource-interface
end
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2
configureterminal
Example:
Router# configure terminal
Enters global configuration mode.
Step 3
ipdhcprelayinformationoption
Example:
Router(config)# ip dhcp relay information option
Enables the DHCP option 82 support on relay agent.
Enabling the DHCP option 82 support allows the system to insert the DHCP relay agent information option in forwarded BOOT
REQUEST messages to a Cisco IOS DHCP server.
Step 4
rbenasipsource-interface
Example:
Router(config)# rbe nasip loopback0
Specifies the IP address of an interface on the DHCP relay agent that will be sent to the DHCP server via the Agent Remote
ID suboption.
Step 5
end
Example:
Router(config)# end
Exits global configuration mode and enters privileged configuration mode.
Configuring the DHCP Lease Limit
Perform this task to limit the number of DHCP leases allowed on ATM RBE unnumbered or serial unnumbered interfaces.
SUMMARY STEPS
enable
configureterminal
ipdhcplimitleaseperinterfacelease-limit
end
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2
configureterminal
Example:
Router# configure terminal
Enters global configuration mode.
Step 3
ipdhcplimitleaseperinterfacelease-limit
Example:
Router(config)# ip dhcp limit lease per interface 2
Limits the number of leases offered to DHCP clients behind an ATM RBE unnumbered or serial unnumbered interface.
Step 4
end
Example:
Router(config)# end
Exits global configuration mode and returns to privileged EXEC mode.
Troubleshooting the DHCP Lease Limit
Perform this task to troubleshoot the DHCP lease limit.
SUMMARY STEPS
enable
debugipdhcpserverpacket
debugipdhcpserverevents
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2
debugipdhcpserverpacket
Example:
Router# debug ip dhcp server packet
(Optional) Decodes DHCP receptions and transmissions.
Step 3
debugipdhcpserverevents
Example:
Router(config)# debug ip dhcp server events
(Optional) Displays server events.
Configuration Examples for Providing Connectivity Using ATM Routed Bridge Encapsulation
The following examples show various ways to provide connectivity from a remote bridged network to a routed network using ATM
RBE.
Example Configuring ATM RBE on PVCs
The following example shows a typical ATM routed bridge encapsulation configuration:
enable
configure terminal
interface atm 4/0.100 point-to-point
ip address 209.165.200.225 255.255.255.224
pvc 0/32
end
Example Configuring ATM RBE on an Unnumbered Interface
The following example uses a static route to point to an unnumbered interface:
enable
configure terminal
interface loopback 0
ip address 209.165.200.226 255.255.255.224
interface atm 4/0.100 point-to-point
ip unnumbered loopback 0
pvc 0/32
atm route-bridge ip
exit
ip route 209.165.200.228 255.255.255.224 atm 4/0.100
end
Example Concurrent Bridging and ATM RBE
The following example shows concurrent use of ATM RBE with normal bridging. IP datagrams are route-bridged, and other protocols
(such as IPX or AppleTalk) are bridged.
bridge 1 protocol ieee
interface atm 4/0.100 point-to-point
ip address 209.165.200.225 255.255.255.224
pvc 0/32
bridge-group 1
atm route-bridge ip
Example DHCP Option 82 for RBE Configuration
In the following example, DHCP option 82 support is enabled on the DHCP relay agent using theipdhcprelayinformationoption command. Therbenasip command configures the router to forward the IP address for Loopback0 to the DHCP server.
ip dhcp-server 209.165.200.225
!
ip dhcp relay information option
!
interface Loopback0
ip address 209.165.201.0 255.255.255.248
!
interface atm 4/0
no ip address
!
interface atm 4/0.1 point-to-point
ip unnumbered Loopback0
ip helper-address 209.165.201.3
atm route-bridged ip
pvc 88/800
encapsulation aal5snap
!
!
interface Ethernet5/1
ip address 209.165.201.4 255.255.255.248
!
router eigrp 100
network 209.165.201.0
network 209.165.200.0
!
rbe nasip Loopback0
For the configuration example, the value (in hexadecimal) of the agent remote ID suboption would be 010100000B01018140580320.
The table below shows the value of each field within the agent remote ID suboption.
Table 2. Agent Remote ID Suboption Field Values
Agent Remote ID Suboption Field
Value
Port Type
0x01
Version
0x01
Reserved
undefined
NAS IP Address
0x0B010181 (hexadecimal value of 11.1.1.129)
NAS Port
Interface (slot/module/port)
VPI
VCI
0x40 (The slot/module/port values are 01 00/0/000.)
0x58 (hexadecimal value of 88)
0x320 (hexadecimal value of 800)
Example DHCP Lease Limit
In the following example, if more than three clients try to obtain an IP address from interface ATM4/0.1, the DHCPDISCOVER
packets will not be forwarded to the DHCP server. If the DHCP server resides on the same router, DHCP will not reply to more
than three clients.
ip dhcp limit lease per interface 3
!
interface loopback0
ip address 209.165.201.3 255.255.255.248
!
interface atm 4/0.1
no ip address
!
interface atm 4/0.1 point-to-point
ip helper-address 172.16.1.2
ip unnumbered loopback0
atm route-bridged ip
pvc 88/800
encapsulation aal5snap
Additional References
Related Documents
Related Topic
Document Title
Cisco IOS commands
Broadband Access Aggregation and DSL commands
Cisco IOS Broadband Access Aggregation and DSL Command Reference
Broadband access aggregation concepts
Understanding Broadband Access Aggregation
Preparing for broadband access aggregation task
Preparing for Broadband Access Aggregation
DHCP commands
Cisco IOS IP Addressing Services Command Reference
DHCP configuration tasks
"Configuring the Cisco IOS DHCP Server" module in the
Cisco IOS IP Addressing Services Configuration Guide
Standards
Standards
Title
None
--
MIBs
MIBs
MIBs Link
None
To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found
at the following URL:
The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and
resolving technical issues with Cisco products and technologies.
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Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.
Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.
Feature Information for Providing Connectivity Using ATM Routed Bridge Encapsulation
The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco
Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 3. Feature Information for Providing Connectivity Using ATM Routed Bridge Encapsulation
Feature Name
Releases
Feature Information
Bridged 1483 Encapsulated Traffic over ATM SVCs
12.4(15)T 12.2(33)SRE
The Bridged 1483 Encapsulated Traffic over ATM SVCs feature provides support for bridged 1483 encapsulated packets to trigger
ATM SVC and also support for sending this traffic on triggered ATM SVCs.
DHCP Option 82 Support for Routed Bridge Encapsulation
15.1(1)S 12.2(2)T
This feature provides support for the DHCP relay agent information option when ATM RBE is used.
The following command was introduced:rbenasip
DHCP Lease Limit per ATM RBE Unnumbered Interface
12.3(2)T
This feature limits the number of DHCP leases per subinterface offered to DHCP clients connected from an ATM RBE unnumbered
interface or serial unnumbered interface of the DHCP server or DHCP relay agent.
The following command was introduced:ipdhcplimitleaseperinterface
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