[c-nsp] Layer 3 Etherchannel and MPLS MTU

gladston at br.ibm.com gladston at br.ibm.com
Sun Feb 26 07:40:57 EST 2006


If two 7609 are connected through layer 3 giga etherchannel and 7609-1 
sends a frame with 1514 bytes plus two MPLS header to 7609-2, does the 
command “mpls mtu 1522” under the giga etherchannel interface on both 
routers is enough? 
Or it is necessary to add the global command “system jumbomtu 1522” and 
the interface command “mtu 1522” under physical gigainterfaces 
participating on the channel?

Cisco docs say many platforms will accept frames higher than 1500 bytes 
(up to 24 bytes) to accommodate control flags; that is why I am 
considering just “mpls mtu xxx” is enough.

Also, reading Cisco doc I understood that “system jumbomtu xxxx” and “mtu 
xxxx” should be used together. I tested sending frames higher than 1514 
bytes (without MPLS tags) and “system jumbomtu xxxx” alone was not enough, 
ping failed; it was necessary to add “mtu xxxx” under the interface. Do 
you have a different experience?


Alaerte Gladston Vidali
IBM Global Services - SO
Tel.55+11+2121-2879   Fax:55+11+2121-2449

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