Hey Jay, just curious if you can explain a bit further, but if there are two 802.1q tags, taking up 4 bytes each, should the system MTU be set to 1508? Also, say you have two customers each using VLAN 12 and they both want to be trunked across the service provider network, you would have to use different "access" VLAN's for the QinQ on the the customer facing interfaces, right? Say VLAN 99 for customer 1 and VLAN 100 for customer 2?If I can answer the second question first, yes, that is absolutely correct.
For the first question though my first inclination was to reply saying that the system mtu command is used to specify the layer 3 payload, not the layer 2 payload (since the default is 1500 bytes). This line of thought was due to the logic of the L2 MTU being 1518 (1500 plus an 18 byte Ethernet header/trailer) and thinking that switches allow an extra 4 bytes (1522 total) when a Dot 1Q tag is added.
But then I thought about this and the L3 MTU line of thought doesn't add up... If it's the L3 MTU why are we adjusting it for the extra Dot 1Q tag at layer 2? And if the system mtu command is for layer 2 then why is it only 1500 (1504 for QinQ) and not 1518, 1522, or even 1526 for a double tagged frame?
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