CDP

Answered Question
Aug 16th, 2010

Hi All,

          One basic question that i have is

When CDP packets are exchanged between two switches on a trunk link with a native vlan 1, i presume that the CDP packets go untagged with a vlan of 1. Now, when i remove vlan 1 from the trunk link and replace the native vlan with, say, vlan 4. Shouldn't the CDP packets go on vlan 4?

I have configured for monitor session on that trunk port ( with encapsulation so i can see which vlan is CDP part of) and my destination contains a pc with a realtek card with which i can see the tagged packets from those mirrored ports.

Even after I remove Vlan 1 and replace the native vlan as 4, i still see my CDP packets tagged with vlan 1. Any reason why?

Just to double check i set up an ip address for vlan 4 on both the switches and sent out an ICMP from one to the another. I could see that the ICMP packets were correctly tagged as vlan 4.

-/ Kiran Cherian

I have this problem too.
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Correct Answer by Yudong Wu about 6 years 5 months ago

Vlan 1 could not be fully removed. Even if vlan 1 is pruned from trunk, the control packet will be still sent in vlan 1. This is expected behavior in Cisco switch.

Correct Answer by Yudong Wu about 6 years 5 months ago
Control protocols like CDP, VTP, PAGP are always sent on vlan 1 for cisco switches no what what native vlan is.
If vlan 1 is not native vlan, those control packet will be tagged as vlan 1. V
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Correct Answer
Yudong Wu Mon, 08/16/2010 - 20:29
Control protocols like CDP, VTP, PAGP are always sent on vlan 1 for cisco switches no what what native vlan is.
If vlan 1 is not native vlan, those control packet will be tagged as vlan 1. V
cherian.kiran Mon, 08/16/2010 - 20:33

Hello Wu,

                 Thank you for the reply. But there is a case in my scenario where in i have REMOVED vlan 1 from my trunk link and configured Vlan 4 as my native vlan. If as you mentioned, CDP packets always go through Vlan1 no matter what my native vlan is, then i should not be seeing CDP packets at all at the other end.

Am i right?

-/ Kiran Cherian

Correct Answer
Yudong Wu Mon, 08/16/2010 - 20:37

Vlan 1 could not be fully removed. Even if vlan 1 is pruned from trunk, the control packet will be still sent in vlan 1. This is expected behavior in Cisco switch.

cherian.kiran Mon, 08/16/2010 - 20:41

Hmmm makes sense as to why the packets go as Vlan 1. Also, just an addition to the same thread, The ADDRESS TLV of CDP, on what basis or rather which Ip address does the switch pick up for it's TLV before seding out the CDP to it's neighbors?

For example, if i have about 5 Vlans (1 to 5)on the 3560G and except Vlan 1 all have ip addresses. Which ip address will be chosed to be sent out in it's TLV?

Thank you for the help.

-/ Kiran

Yudong Wu Mon, 08/16/2010 - 20:55

If I remember correctly, IP address with lowest vlan ID should be used in CDP nei.

You can verify it by a testing. HTH.

cherian.kiran Mon, 08/16/2010 - 21:01

Hi Wu,

             Yup.I will test that out too and check.

Thank you for all the help.

-/ Kiran Cherian

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