[c-nsp] Cisco 3550 MTU

[Tom Storey]

Hi all.

The maximum configurable MTU for a 3550 series (excl 3550-12's which  
can do 2000) is 1546 bytes. Can anyone confirm if this is PAYLOAD  
size, or raw frame size?

I am interested in using a number of these in a network "upgrade"  
where MPLS is required.

My specific reasoning behind wanting to know is in relation to  
pseudowires and the ability to transport a full 1500 byte IP packet  
without having to resort to fragmentation.

A pseudowire transporting an IP packet in tagged ethernet over tagged  
ethernet needs (in its most basic form):

14 (outer ethernet) + 4 (outer VLAN) + 8 (2 MPLS labels) + 4 (PW  
control word) + 14 (inner ethernet) + 4 (innter VLAN) + 1500 IP packet

Thats 1548 bytes. 2 bytes over blown (sounds a lot like the budget I  
have to accomplish this upgrade). :-)

But if 1546 is meerly the payload I can transport, and if ontop of  
that are the outer ethernet headers (e.g. 1564 raw frame size) then  
all is fine and dandy, and I get a few more bytes to play with in  
customer ethernet/MPLS land.

Anyone got any comments from prior experience with these switches?

Thanks,
Tom