One-way CDP neighborship

Unanswered Question
Jan 4th, 2008
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On trunks between some routers and switches, the routers are visible on the switches, but the routers cannot see the switches in CDP. The trunks are otherwise working fine.

It looks like the routers only supports cdp v1, so I've tried to disable v2-advertising on the switches as well.

Any ideas?


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ankbhasi Fri, 01/04/2008 - 06:59
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Hi Friend,

Can you please confirm if cdp is not disable on switch interface on which router is connected? CDP can be enabled globally and can be disabled on interface level as well.



akorsvoll Fri, 01/04/2008 - 07:11
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Thanks for answering

CDP is enabled on all routerinterfaces. The router can see other routers on other interfaces. This also occurs on routers supporting cdp v2.

Can there be an issue with CDP and trunking/native-VLAN?



ankbhasi Fri, 01/04/2008 - 07:17
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Hi Anders,

As you mentioned in your original post routers are visible on switches means router is sending CDP information and switch reveives and displays but routers cannot see switches and what I am presuming is CDP can be disabled on switch interface level.

If CDP is enabled on the switch interfaces also is it possible for you to sniff the switch interface and see if switch is sending CDP information?



akorsvoll Fri, 01/04/2008 - 14:10
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Hi Ankur

Sorry i didn't answer your last question completely.

CDP is enabled on every switch-trunk interface, and CDP is working fine on interswitch-links. The problem seems to be only on the routers. I have no doubt that CDP is enabled, and hope to avoid sniffing the interfaces as long as possible.

Thanks again,


j-marenda Fri, 01/04/2008 - 09:09
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cdp (and udld) uses the "native" untagged VLAN. You may want to create a Gig/FastEthernet subinterface just for cdp :

interface GigabitEthernet0/1.1

description tpk-vlan1-cdp-only

encapsulation dot1Q 1 native


dabels Fri, 01/04/2008 - 13:58
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is it possible to get the config for the router interfaces and the swith port?

akorsvoll Fri, 01/04/2008 - 14:15
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This seems reasonable and I will try this on the routers that are supporting this command.

Some routers however are running IOS 12.0.(xx).S which doesn't seem to accept the keyword native.




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