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How one Switch identify the Native vlan mismatch

Sanjib Pradhan
Level 1
Level 1

Dear All,

I am using two cisco L2 switches. Both are connected by a trunk link. Unfortunately I configured different native vlan between two switches. Suddenly I got an error that native vlan mismatch. When I changed the configuration Now it's working fine. My question is that how one switch identify that native vlan mismatch(either by Bpdu, cdp or packet). Please mention which of the following used by switch to identify native Vlan mismatch.

Regards,

Sanjib

5 Replies 5

It's CDP. Look at the output of "sh cdp neighbors detail" and you see that the native VLAN ist transmitted there.

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Sanjib, Karsten,

It's CDP.

Yes, and STP as well if you run a trunk between the two switches. PVST+ and RPVST+ BPDUs have a TLV in their trailer that carries the VLAN number for which the BPDU was originated. If the BPDU is received in a different VLAN (caused by a native VLAN mismatch), the receiving switch will be able to detect it.

Wireshark 1.12.x will be capable of displaying this TLV field in captured PVST+ and RPVST+ BPDUs. Until 1.12.x is released, you may want to try daily builds from:

http://www.wireshark.org/download/automated/

They already incorporate the enhancement.

Best regards,

Peter

thanks for the clarification, I wasn't aware of that. Good to know!

-- 
Don't stop after you've improved your network! Improve the world by lending money to the working poor:
http://www.kiva.org/invitedby/karsteni

Peter,

Does CDP detect it as well or does it just inform about the misconfiguration by learning it via STP? So if STP is disabled would CDP still detect it by itself?

Daniel Dib
CCIE #37149

Please rate helpful posts.

Daniel Dib
CCIE #37149
CCDE #20160011

Please rate helpful posts.

Hi Daniel,

CDP performs the detection on its own, independently of STP. If you disabled STP, CDP would still detect the mismatch and report it. However, as opposed to STP, CDP would take no action, while STP would block the offending pair of VLANs.

The STP-based detection works only on trunks where BPDUs for multiple VLANs are potentially sent and received. If the link interconnecting the switches operated as an access link, STP would use a standard BPDU format without the ability to detect native VLAN mismatches.

Best regards,

Peter

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