CDP is primarily used to obtain protocol addresses of
neighboring devices and discover the platform of those devices. CDP can also be
used to display information about the interfaces your router uses. CDP is
media- and protocol-independent, and runs on all equipment manufactured by
Cisco, including routers, bridges, access servers, and switches.
Use of SNMP with the
CDP MIB allows network management applications to learn the device type and the
SNMP agent address of neighboring devices and to send SNMP queries to those
devices. CDP uses the CISCO-CDP-MIB.
CDP runs on all media
that support Subnetwork Access Protocol (SNAP), including LAN, Frame Relay, and
ATM physical media. CDP runs over the data link layer only. Therefore, two
systems that support different network-layer protocols can learn about each
other.
Each device configured
for CDP sends periodic messages, known as
advertisements, to a multicast address. Each device
advertises at least one address at which it can receive SNMP messages. The
advertisements also contain time-to-live, or hold-time, information, which
indicates the length of time a receiving device holds CDP information before
discarding it. Each device also listens to the periodic CDP messages sent by
others to learn about neighboring devices and determine when their interfaces
to the media go up or down.
CDP Version-2 (CDPv2)
is the most recent release of the protocol and provides more intelligent device
tracking features. These features include a reporting mechanism that allows for
more rapid error tracking, thereby reducing costly downtime. Reported error
messages can be sent to the console or to a logging server, and can cover
instances of unmatching native VLAN IDs (IEEE 802.1Q) on connecting ports, and
unmatching port duplex states between connecting devices.
CDPv2
show
commands can provide detailed output on VLAN Trunking Protocol
(VTP) management domain and duplex modes of neighbor devices, CDP-related
counters, and VLAN IDs of connecting ports.
Type-length-value fields (TLVs) are blocks of information embedded in CDP advertisements. This table summarizes the TLV definitions
for CDP advertisements.
Table 1. Type-Length-Value
Definitions for CDPv2
TLV
|
Definition
|
Device-ID TLV
|
Identifies the
device name in the form of a character string.
|
Address TLV
|
Contains a
list of network addresses of both receiving and sending devices.
|
Port-ID TLV
|
Identifies the
port on which the CDP packet is sent.
|
Capabilities
TLV
|
Describes the
functional capability for the device in the form of a device type; for example,
a switch.
|
Version TLV
|
Contains
information about the software release version on which the device is running.
|
Platform TLV
|
Describes the
hardware platform name of the device, for example, Cisco 4500.
|
VTP Management
Domain TLV
|
Advertises the
system’s configured VTP management domain name-string. Used by network
operators to verify VTP domain configuration in adjacent network nodes.
|
Native VLAN
TLV
|
Indicates, per
interface, the assumed VLAN for untagged packets on the interface. CDP learns
the native VLAN for an interface. This feature is implemented only for
interfaces that support the IEEE 802.1Q protocol.
|
Full/Half
Duplex TLV
|
Indicates
status (duplex configuration) of CDP broadcast interface. Used by network
operators to diagnose connectivity problems between adjacent network elements.
|