IP address in Show CDP Neighbor Detail command

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Aug 4th, 2010
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When you do a Show cdp neighbor detail on a distribution layer switch or a router where there are more than one IP address, what is the principle that the protocol used to show its IP address to its neighor?


thanks,


Han

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Collin Clark Wed, 08/04/2010 - 08:20
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I believe it's the closest interface IP. You can change the default action with source cdp-source .

Collin Clark Wed, 08/04/2010 - 08:32
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If you're on router A and router B is connected via a T1 link, when you look at CDP, the IP address assigned to the serial interface on router B is the closest. The ethernet on router B is farther away since it has to cross the serial interface.

Sebastian Helmer Wed, 08/04/2010 - 08:53
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Hi


I would say until here the answers are right.


When you use SVI's  you can't define the interface that should use CDP like I know. In my enviroment it seems that cdp use the SVI with the biggest subnet, but is just a thought.


I would say, if you want define that you can only disable cdp at all and enable it only on the svi and mabye necessary on the physikal ports you want.


best regards

Sebatian


PS: If I have the time I play a little bit with that tomorrow...

hanwucisco Wed, 08/04/2010 - 09:15
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I had two exactly same secarios:


Switch===trunk===subinterfaces on router;


where when I do show cdp nei detail on the switch side, one shows the sub-interface for the Management vlan in the VTP domain, whereas, one shows a giga0/0 interface ip which is not related to the subinterfaces.


any idea?


thanks,

Sebastian Helmer Wed, 08/04/2010 - 09:30
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quit interessting, can you show me the output (with changes ip's or x.x.x.x)

hanwucisco Wed, 08/04/2010 - 10:50
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switch1#sh cdp nei gi0/12 d
-------------------------
Device ID: ABC
Entry address(es):
  IP address: X.X.X.129
Platform: Cisco 3845,  Capabilities: Router Switch IGMP
Interface: GigabitEthernet0/12,  Port ID (outgoing port): GigabitEthernet0/1.7
Holdtime : 178 sec

Version :
Cisco IOS Software, 3800 Software (C3845-ADVIPSERVICESK9-M), Version 12.4(15)T10, RELEASE SOFTWARE (fc3)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2009 by Cisco Systems, Inc.
Compiled Mon 14-Sep-09 17:29 by prod_rel_team

advertisement version: 2
VTP Management Domain: ''
Duplex: full
Management address(es):



Gi0/1.7 and its IP X.X.X.129, I use as the management Vlan gateway.


=============================================


Switch2#sh cdp nei gi1/0/23 d
-------------------------
Device ID: SSS

Entry address(es):
  IP address: A.B.C.2

Platform: Cisco 3845,  Capabilities: Router Switch IGMP
Interface: GigabitEthernet1/0/23,  Port ID (outgoing port): GigabitEthernet0/1
Holdtime : 169 sec

Version :
Cisco IOS Software, 3800 Software (C3845-ADVIPSERVICESK9-M), Version 12.4(15)T10, RELEASE SOFTWARE (fc3)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2009 by Cisco Systems, Inc.
Compiled Mon 14-Sep-09 17:29 by prod_rel_team

advertisement version: 2
VTP Management Domain: ''
Duplex: full
Management address(es):


---------

IP A.B.C.2 is on interface gi0/0 at Router SSS, the link is on gi0/1. It doesnt use subinterface .2, which i use for management vlan.

Sebastian Helmer Wed, 08/04/2010 - 11:13
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mmhhh,

I understand right? (mabye it is to late for me )

2 Switches 1 Router. Both switche are connected to the same router? one cdp output looks "ok", one output looks strange because it shows Gig 0/0 which is not used for the trunk.


anyway. when I'm wrong and you speak about two different pairs. pair 1 (router A and switch A looks "ok") and pair 2( router B and switch B looks strange). I think interessting would be details of the router subinterfaces and if all vlan*s are allowed on the trunk.


But I need to be fair, i have not really an idea but it is also different for me to understand you enviroment. I think the answer will be in a detail I not know in the moment. (Draw and config would be help )

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