[c-nsp] CDP interoperability

[harbor235]

Aivars,

Best practice would be to remove VLAN 1 from the list of trunked VLANs.



Mike

On Mon, Dec 10, 2012 at 10:39 AM, Aivars <aivars at ml.lv> wrote:

>   Hi,
>
>   I thought that CDP essence is to help understand what device you
>   have at the other end of the wire no matter what. You just plug one
>   end of the cable into one box and the other end into another and
>   you get your CDP neighbors. Besides other side usage like in IP
>   phone communication with switches this is why anybody would use CDP.
>   Right?
>
>   Up until this morning I also thought that CDP frames are always sent
>   untagged. This is the way I would do it. Well, I was wrong. Actually
>   on Catalyst switches CDPs are sent in vlan 1. If you make some other
>   vlan native on a trunk port, CDPs are sent with dot1q tag "1". vlan
>   dot1q tag native will also do the same trick.
>
>   Now imagine a brand new shiny IOS-XR box, ASR9k for example. If it
>   has no subinterface configuration with encapsulation dot1q 1, CDP
>   will be broken. It will send CDPs with no tag and Catalyst will be
>   happy about it. It will show ASR as CDP neighbor. ASR instead
>   doesn't now what a hell tag "1" means and drop these frames.
>
>   Cisco thinks - this is expected behavior.
>
>   What do you guys think? Is this a bug or a feature? Should it remain
>   as it is?
>
>   Aivars
>
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