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Information About Implementing CDP
Information About Implementing CDP
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.
Type-Length-Value Definitions for CDPv2, on page 98
advertisements.
Table 13: Type-Length-Value Definitions for CDPv2
TLV
Device-ID TLV
Address TLV
Port-ID TLV
Capabilities TLV
Version TLV
Platform TLV
System Management Configuration Guide for the Cisco NCS 6000 Series Router, Release 5.0.x
98
summarizes the TLV definitions for CDP
Definition
Identifies the device name in the form of a character string.
Contains a list of network addresses of both receiving and sending
devices.
Identifies the port on which the CDP packet is sent.
Describes the functional capability for the device in the form of a
device type; for example, a switch.
Contains information about the software release version on which the
device is running.
Describes the hardware platform name of the device, for example,
Cisco 4500.
Implementing CDP
Table 13:
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