[c-nsp] ISIS and MTU mismatch

[Jason Lixfeld]

I've been doing some experimenting with MTU size mismatches and ISIS adjacencies.

Consider an ME3400 SVI facing a 7600 ES port subinterface:

[ME3400 SVI -> Gi0/1] --- [7600 ES Gi7/0/19 -> Gi7/0/19.68]

The default MTU on the ES port is 1500, which the subinterface inherits.

The MTU on the ME3400 SVI is based on the system MTU size, or 1998:

! ME3400
me3400#show system mtu 

System MTU size is 1998 bytes
System Jumbo MTU size is 9000 bytes
System Alternate MTU size is 1998 bytes
Routing MTU size is 1500 bytes

The 7600/1500 to ME3400/1998 mismatched MTU works fine.  ISIS adjacency forms, no problem.

If I leave ME3400/1998 and set 7600/9000, the ISIS adjacency gets torn down.

Why would a bigger >  smaller MTU work fine in one direction (ME3400 > 7600) but not the other direction (7600 > ME3400)?