MTU setting differences between 7600 and ME3750

30 09 2008

Just for personal reference really:

The ME3750 has several ways of setting the MTU:

system mtu <size>

Sets the ethernet MTU of the ports and can go up to 1998 bytes.

system mtu jumbo <size>

Sets the ethernet MTU on the gigabit ports up to 9000 bytes.  This overrides the previous command for these ports.  So if you have both set, the 10/100 ports will have one value (e.g. 1560) and the gigabit ports will have another (e.g. 9000).

system mtu routing <size>

Sets the MTU for protocols run on this unit.  It depends on the system mtu being set already. If you are running OSPF, and your neighbouring routers don’t agree on this MTU size, you will not form neighbour relationships (unless you configure “ip ospf mtu-ignore” on the interfaces)

On the 7600, if you do an “mtu 1560” on an interface, it seems that both the IP and the ethernet MTU change to the new value.

Of course Cisco actually ignore the ethernet headers when they are talking about ethernet MTU sizes.  So a byte-size of 1500 actually ignores the 18 bytes of ethernet headers.  It therefore makes the ethernet MTU the same as the IP MTU (by default) so it isnt’ surprising that there’s some confusion…

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