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Appendix B - Cisco Discovery Protocol (CDP)

Cisco & IP Addressing
Louis D. Rossi, Louis R. Rossi and Thomas Rossi
  Copyright © 1999 The McGraw-Hill Companies, Inc.

Appendix B: Cisco Discovery Protocol (CDP)
Overview
I have included CDP because this Cisco protocol can prove to be a very valuable tool to help troubleshoot layer 3 addressing problems. Also CDP can be used to document a Cisco network topology. CDP is media and protocol-independent which makes this protocol so valuable. Let us assume that you have a layer 3 addressing problem, say for sake of argument, a misconfigured address on a router or a switch.
From a neighboring device we can view the layer 3 addressing with out having layer 3 connectivity.
In the example below I have intentionally addressed Router_B incorrectly, even so Router_A, using CDP, can view the addressing of Router_B.
Router_A>en
Router_A#sh ip int s0
Serial0 is up, line protocol is up
  Internet address is 172.16.4.5/30
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
Helper address is not set
    Directed broadcast forwarding is enabled
    Multicast reserved groups joined: 224.0.0.9
    Outgoing access list is not set
    Inbound  access list is not set
    Proxy ARP is enabled
    Security level is default
    Split horizon is enabled
    ICMP redirects are always sent
    ICMP unreachables are always sent
    ICMP mask replies are never sent
    IP fast switching is enabled
    IP fast switching on the same interface is enabled
    IP multicast fast switching is disabled
    Router Discovery is disabled
    IP output packet accounting is disabled
    IP access violation accounting is disabled
    TCP/IP header compression is disabled
Router_A#sh cdp neighbors detail
----------------------------------------------------------
Device ID: Router_B
Entry address(es):
  IP address: _134.16.16.5 <this address should be on the 172.16.4.4 subnet>
Platform: cisco 2521,  Capabilities: Router
Interface: Serial0,  Port ID (outgoing port): Serial0
Holdtime : 172 sec
Version :
Cisco Internetwork Operating System Software
IOS (tm) _2500 Software (C2500-JS-L), Version 11.3(1), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-1997 by cisco Systems, Inc.
Compiled Mon 15-Dec-97 18:28 by richardd
————————————-
Device ID: receive_ntp
Entry address(es):
  IP address: 172.16.4.6
Platform: cisco 2521,  Capabilities: Router
Interface: Serial0,  Port ID (outgoing port): Serial0
Holdtime : 52 sec
Version :
Cisco Internetwork Operating System Software
IOS (tm) _2500 Software (C2500-JS-L), Version 11.3(1), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-1997 by cisco Systems, Inc.
Compiled Mon 15-Dec-97 18:28 by richardd
Figure B-1  CDP results
When you suspect there may be an addressing problem us the “show cdp” neighbor command to view the addressing on the other side of the link.
CDP can also be used to document a Cisco network. I had a student a while back bring to me a beautiful diagram of his 5000 node network. The diagram was actually put together by a relatively non-technical person. By using CDP he could discover all the directly connected platforms. Then by telneting to the next router another CDP discovery could be performed. He continued in this manner to construct a diagram of the entire network.

 


 
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