The Cisco Discovery Protocol (CDP) is a Cisco proprietary Data Link Layer network protocol used to share information about other directly connected Cisco equipment. CDP can also be used in place of dynamic routing protocols in simple networks. Cisco devices send CDP announcements out each connected network interface to the multicast destination address 01-00-0C-CC-CC-CC. By default CDP announcements are sent every 60 seconds. The “show cdp neighbors” command that you are probably familiar with basically displays stored information that has been collected from other devices. Further, the CDP data is also accessible via snmp. The holdtime, which specifies the lifetime of an entry in the table, by default is 180 seconds.
The “cdp advertise-v2″ global configuration command can be used to enable Cisco Discovery Protocol Version 2 (CDPv2) advertising functionality on a device. However, it is already on by default.
Here is a snapshot of the command run on a Cisco 2621XM router [IOS version 12.3 (16a)]:
Interestingly, Hewlett-Packard leased the CDP protocol from Cisco but starting from products shipped after February 2006 has removed support for transmitting CDP from HP Procurve switches; however, receiving and processing CDP information is still supported. CDP support was replaced with IEEE 802.1AB Link Layer Discovery Protocol, an IEEE standard that is functionally similar to CDP. This is the protocol used by Force10 switches. However, unlike Cisco, the protocol is not enabled by default and must be enabled as shown below on a Force10 S60 switch/router [FTOS 126.96.36.199]:
Once enabled, you can use “show lldp neighbors” or “show lldp statistics” in privileged exec mode. I don’t have any neighbors setup with this switch/router so below I only display the options available.