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Attention: The Cisco Support Community site will be in read only mode on Dec14, 2017 from 12:01am PST to 11:30am for standard maintenance. Sorry for the inconvenience.

New Member

CDP Help

Router1: 168.206.8.1

Router2: 168.206.8.65

Router3: 168.206.8.202

Router 1 and 2 exchange CDP info.

Router3 show cdp traf: Total packets output: 1036, Input: 0

All the router can ping each other. Any ideas why router3 doesn't show any neigh

1 ACCEPTED SOLUTION

Accepted Solutions

Re: CDP Help

You need to look at your switches - I would put money on them not forwarding multicast traffic correctly. Am I right in an assumption (given use of plural and behaviour of CDP) that R1&R2 are in the same switch, with R3 in another? If so the first place to look is the inter switch link.

I am also a ittle concerned that R2 appears ro be showing itself in CDP (again assumption yyy=r2)

You really need to be looking at the switches.

9 REPLIES
Cisco Employee

Re: CDP Help

Hi Friend,

Can you check if cdp is not disabled on interface of router 2 which is connected to router 3?

Regards,

Ankur

New Member

Re: CDP Help

Hi Ankur

CDP is enabled on all the routers

R1/R2/R3: show cdp

Global CDP information:

Sending CDP packets every 60 seconds

Sending a holdtime value of 180 seconds

Sending CDPv2 advertisements is enabled

Cisco Employee

Re: CDP Help

Hi Friend,

CDP can be enabled globally and also on interface level. It may be enabled globally on router 2 but can be disabled on per interface on which router 3 is connected?

run a command "sh run int " on router interface and check to confirm.

Regards,

Ankur

Re: CDP Help

How are these connected? Superficially the appear to be in the same subnet, but that is not necessarily the case, as the use of .1 and .65 suggests those may be on /26 subnets.

*assuming* they are on one subnets, then the chances are they are all on a switch. Can 1&2 see 3? If 1&2 can see each other but R3 neither sees nor is seen, then there is potentially an issue with L2 multicast traffic to/from R3.

We need to know more about the interconnections between the routers, and the configs of any switches - are they Cisco switches? I have seen other switches cause problems with CDP.

New Member

Re: CDP Help

Hi P

R1: 168.206.8.1/21 Eth0 (1605)

R2: 168.206.8.65/21 FA0 (1721)

R3: 168.206.8.201/1 FA0/1 (1841)

R1/2 & 3 is connected via ethernet cable and 3Com 4400 switches

Router 1: show cdp neighb

yyy Eth 1 159 R 1721 Fas 0

Router 2: show cdp neighb

yyy Ser 0 120 R 1721 Ser 0

zzz Fas 0 158 R 1605 Eth 1

Router 3: show cdp neigh: no output

3: show cdp traf

Total packets output: 1258, Input: 0

Hdr syntax: 0, Chksum error: 0, Encaps failed: 0

No memory: 0, Invalid packet: 0, Fragmented: 0

CDP version 1 advertisements output: 0, Input: 0

CDP version 2 advertisements output: 1258, Input: 0

Re: CDP Help

You need to look at your switches - I would put money on them not forwarding multicast traffic correctly. Am I right in an assumption (given use of plural and behaviour of CDP) that R1&R2 are in the same switch, with R3 in another? If so the first place to look is the inter switch link.

I am also a ittle concerned that R2 appears ro be showing itself in CDP (again assumption yyy=r2)

You really need to be looking at the switches.

New Member

Re: CDP Help

thanks Pall

Will do so.

New Member

Re: CDP Help

Side note... wont cdp only show directly connected cisco devices??? if the routers are all connected to a L2 switch why would any be cdp neighbors... wouldnt they be neighbors of the L2 switch, which in this case is a 3COM...

Re: CDP Help

CDP can be fun. CDP will go one Cisco hop, and would normally traverse other vendors at L2.

Take a 3com switch that will forward multicast and plug six cisco switches into it. All Cisco switches will see five neighbours as the 3com has no interest in CDP, to the 3Com it is simply L2 multicast to be forwarded out all ports.

P.

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