[c-nsp] System MTU on trunks for Q in Q

[Peter Rathlev]

On Wed, 2008-03-26 at 19:01 -0400, Dan Armstrong wrote:
> I've been bashing my head against the wall all day for a definitive 
> answer on this:
> 
> On a Cisco switch that supports QinQ (3550, 3750, ME3400, 3560 etc)
> 
> 
> What is the _minimum_ value I need to set the system MTU to, to do 
> QinQ?  1504?  1522?  1526?  1546?
> 
> I can't seem to find one concise answer...

I'm not entirely sure what the "system mtu" specifies, i.e. if it's
interface MTUs (typically excluding data link headers) or what.

IPv4 packet = 1500 bytes. Ethernet header = 14 bytes. 802.1q header = 4
bytes. Another one = 4 bytes more. So for "simple" QinQ of "regular"
IPv4 traffic it would be max 1522 bytes of data per packets.

Any special reason not to just raise it to the maximum of 2000 bytes?

Regards,
Peter