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VLAN mismatch on trunk?

I am currently working on a project to cleanup a network core. We have discovered quite a few trunks that have VLANs defined at one end that do not exist on the other end. I have not been able to find supporting documentation on the TAC website to know what impact this has on the network.

I do understand that Native Vlan Mismatch on a trunk basically bridges the natvie vlan on both ends to create one combined L2 broadcast domain.

Also, I understand that a non-trunk vlan mismatch will create one combined L2 broadcast domain.

The Native vlans match on the trunks, just some vlans only exist on one end.

What if anything happens to broadcast traffic on the un-matched VLANS?

These are 6500 and 5500 switches. Some with L3 capabilites, some without.

TIA

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Community Member

Re: VLAN mismatch on trunk?

The Catalyst automatically drops packets of those VLANs that don't exist on the receiving trunk port. At the Catalyst 5000 familiy this is built into hardware, called Colour Blocking Logic (CBL), alo very helpful for Spanning Tree. Don't know what's the name at the Catalyst 6000, but sure it does the same.

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Community Member

Re: VLAN mismatch on trunk?

The Catalyst automatically drops packets of those VLANs that don't exist on the receiving trunk port. At the Catalyst 5000 familiy this is built into hardware, called Colour Blocking Logic (CBL), alo very helpful for Spanning Tree. Don't know what's the name at the Catalyst 6000, but sure it does the same.

Community Member

Re: VLAN mismatch on trunk?

If your link is not a trunk, all that happens is the packets are passed as though they are on the same vlan. A non-trunked VLAN is only a grouping of ports on a switch. It is only when you create a trunk that VLAN tagging and separation occur.

-Bo

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