[c-nsp] ISIS and MTU mismatch

[Randy]

--- On Sun, 3/25/12, Jason Lixfeld <jason at lixfeld.ca> wrote:

> From: Jason Lixfeld <jason at lixfeld.ca>
> Subject: [c-nsp] ISIS and MTU mismatch
> To: cisco-nsp at puck.nether.net
> Date: Sunday, March 25, 2012, 11:31 AM
> 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)?
> 

Hello Jason,
I believe the routed-mtu size on SVI is still 1500 despite system mtu being 1998
When you change default on 7600 to 9000 you are changing the routed-mtu size.
./Randy