CDP Info on Access Port

Unanswered Question
Aug 26th, 2008
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Hello, I have a few questions regarding CDP.

Is it common practice to disable CDP on access ports?

I am seeing CDP entries in Wireshark for the network I am connected to (Cisco_0a:ff:96 CDP/VTP/DTP/PAgP/UDLD). When I drill down into the packet, specifically under the "Cisco Discovery Protocol" field, I see that the Native VLAN is incorrect. The native VLAN for the subnet I am on, should be 50, but Wireshark shows the Native VLAN as 29, which is the subnet I am attached to.

The trunk port of the switch I am connected to has the "switchport trunk native vlan 50" command.

Any idea what would cause this behavior?

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Giuseppe Larosa Tue, 08/26/2008 - 12:58
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Hello Jason,

if the port config contains:

switchport trunk native vlan 50

switchport access vlan 29

what you see is correct : the native vlan is the untagged Vlan when the port is trunking.

other thought :

you may be receveing the CDP packets out the monitor destination port not only the ones coming from the source port (the one mirrored) and this could explain.

See multiple CDP frames if you see on some vlan 29 and on others vlan 50 you are receveing both.

Hope to help


Jason Fraioli Tue, 08/26/2008 - 15:30
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Mark, Thanks for the best practices.

Giuseppe, the trunk port contains "trunk native vlan", the access port contains "access vlan". The trunk and access commands are not on the same port. In other words, the access port I am connected to (fa0/22) is not trunking.

Mark Yeates Tue, 08/26/2008 - 13:26
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It is a common practice to disable CDP on user access ports.




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