CDP tunneling: why doesn't far end switch show near end host as

From: Hobbs (deadheadblues@gmail.com)
Date: Wed Aug 06 2008 - 13:27:58 ART

Hello,

I am trying get my head around a semi-complex protocol tunneling task, but I
have some questions on the basics.

Here's the topology

"Near end host": BB1
"Far end switch": CAT2

[BB1]f0/1-----f0/11[CAT1]f0/23-----dot1q
trunk-----f0/23[CAT2]f0/1-----f0/1[R1]

To get cdp tunneling to work (and it does work) I enable this on CAT1 and
CAT2 access ports:

CAT1:
interface f0/11
 switchport mode access
 switchport access vlan 100
 l2protocol-tunneling cdp
 no cdp enable

CAT2:
interface f0/1
 switchport mode access
 switchport access vlan 100
 l2protocol-tunneling cdp
 no cdp enable

The trunk between CAT1 and CAT2 carries vlan 100 as well as others.

This is what I understand so far:

BB1 sends a multicast CDP packet on it's port to CAT1.
Since CAT1 has cdp tunneling enabled, it forwards it rather than processes
it. Is this correct?
CAT1 sends this packet out all trunk links that have vlan 100 allowed AND
any access ports on VLAN100. Is this correct?
The packet still has the cisco CDP destination MAC. Is this correct?
The packet is dot1q tagged and sent out port fa0/23 and still has cisco CDP
destination MAC address. Is this correct?

Question:

CAT2 doesn't have tunneling on port fa0/23.
Why doesn't CAT2 now process this CDP packet and show BB1 has a CDP neighbor
on fa0/23?
Does trunking override this behavior?
Does CAT2 treat this packet differently because it is a tagged CDP packet?

Thanks,

Blogs and organic groups at http://www.ccie.net

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