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46 Chapter 2: Cisco Internetwork Operating System (IOS) Fundamentals

CDP is a Cisco proprietary protocol; to support forwarding CDP messages over an interface,
that interface must support SNAP headers. Any LAN interface, HDLC, Frame Relay, and ATM
all support CDP. The router or switch can discover Layer 3 addressing details of neighboring
routers--without even configuring that Layer 3 protocol--because CDP is not dependent on
any particular Layer 3 protocol.

CDP discovers several useful details from the neighboring device:

Device Identifier--Typically the host name.

Address list--Network and data link addresses.

Port Identifier--Text that identifies the port, which is another name for an interface.

Capabilities list--Information on what the device does--for instance, a router or switch.

Platform--The model and OS level running in the device.

CDP is enabled in the configuration by default. The no cdp run global command disables CDP
for the entire device, and the cdp run global command re-enables CDP. Likewise, the no cdp
enable interface subcommand disables CDP just on that interface, and the cdp enable
command switches back to the default state of CDP being enabled.

A variety of show cdp command options are available. Example 2-4 lists the output of the
commands, with some commentary following.

Example 2-4

show cdp Command Options

Seville#show cdp neighbor
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater

Device ID Local Intrfce Holdtme Capability Platform Port ID
fred Ser 1 172 R 2500 Ser 1
Yosemite Ser 0.2 161 R 2500 Ser 0.2

Seville#show cdp entry fred
-------------------------
Device ID: fred
Entry address(es):
IP address: 163.5.8.3
Platform: cisco 2500, Capabilities: Router
Interface: Serial1, Port ID (outgoing port): Serial1
Holdtime : 168 sec

Version :
Cisco Internetwork Operating System Software
IOS (tm) 2500 Software (C2500-D-L), Version 12.0(6), RELEASE SOFTWARE (fc1)
Copyright 1986-1999 by cisco Systems, Inc.
Compiled Tue 10-Aug-99 23:52 by phanguye

Seville#show cdp neighbor detail
-------------------------
Device ID: fred
Entry address(es):

ch02.fm Page 46 Monday, March 20, 2000 4:57 PM