[c-nsp] CDP interoperability

Aivars aivars at ml.lv
Mon Dec 10 10:39:47 EST 2012


  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.

  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?


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