odd mtu behavior on qinq sub-interface

[Ahmed Dockrat]

Hi

I've got a weird issue.

I have a cisco 7200 connecting to a cisco 2901 via a telco providing a layer 2 service and trying to get a 1500 MTU end to end, un-fragmented.

If I configure both ends as a single tagged sub-interface, I can get up to 1642byte unfragmented on the link. If I change both side to use a qinq sub-interface using two tags, my MTU is limited to 1496 which is odd as the extra vlan tag is 4bytes only.

My understanding is that I should be able to get 1638 in this case with a double tag interface due to the additional 4 byte overhead from the second tag.

What could possibly cause this

Thanks

[Ahmed Dockrat · Accepted solution]

Managed to find the issue, was linked to bug CSCuo71154 on EHWIC-1GE-SFP-CU wik card.

Moved the link to a built in port on the router and the issue resolved.