native vlan mismatch on trunk

Unanswered Question
Feb 19th, 2008

A new switch has been added and it was configured for native vlan14

This is connected to a current switch.

both interconnect ports have been configured as trunk

However I am seeing a native vlan mismatch.

13w6d: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEt

hernet5/0/1 (1), with Switch GigabitEthernet0/24 (14). (sw1)

The current switch has vlan1 as

the native. I could change the current one or the new one's native vlan.

My question is-If I change the native vlan on the trunk on the older switch..

What impact would this have on other traffic transversing the switch. Is native vlan configured on a port by port ..trunk by port basis?

I have this problem too.
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lamav Tue, 02/19/2008 - 09:30

By default, a dot1q trunk uses vlan 1 as the native vlan. The native vlan is untagged and is meant to support the following:

• CDP, VTP, and PAgP updates are always forwarded on trunks with a VLAN 1 tag. This is the case even if VLAN 1 is cleared from the trunks and is not the native VLAN. If VLAN 1 is cleared for user data, there is no impact on control plane traffic that is still sent using VLAN 1.

• In PVST+, the 802.1Q IEEE BPDUs are forwarded untagged on the common Spanning Tree VLAN 1 for interoperability with other vendors, unless VLAN 1 is cleared from the trunk. This is the case regardless of the native VLAN configuration. Cisco PVST+ BPDUs are sent and tagged for all other VLANs.

• 802.1s Multiple Spanning Tree (MST) BPDUs are always sent on VLAN 1 on both ISL and 802.1Q trunks. This applies even when VLAN 1 is cleared from the trunks.

• Do not clear or disable VLAN 1 on trunks between MST bridges and PVST+ bridges. But, in the case that VLAN 1 is disabled, the MST bridge must become root in order for all VLANs to avoid the MST bridge putting its boundary ports in the root-inconsistent state.

To answer your question, I would change the native vlan on the new switch to the default vlan 1 setting for dot1q trunks. And yes, it is done on a per-port basis.



lamav Tue, 02/19/2008 - 09:37

By the wya, ifd you ever want to create an etherchannel from a grouping of trunk ports, all the ports must have the native vlan configured the same way...



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