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

[Tassos Chatzithomaoglou]

Since 1500 is the default and 1504 is by default "supported" on 802.1q trunk links, i guess 1504 
should be the correct value for 802.1q tunneling. I guess the ethernet header is not counted by default.

My proposition? Use something that covers all of them (i.e. 1600 for GE, 1546 for FE) and (as long 
as you don't change the routing MTU if you also use the switch as a L3 device ) you'll be fine... 
for many years.

I still haven't found any reason for keeping a low MTU on L2 switches (although i don't know if any 
L2 protocols can generate such large "frames" which could possibly get dropped in a 1500 link).

--
Tassos


Dan Armstrong wrote on 27/3/2008 1:01 πμ:
> 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...
> 
> 
> Thanks!!
> 
> 
> 
> 
> 
> 
> 
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