MTU, Junos & Cisco, OSPF

Here’s a frustrating thing for people working with multiple hardware vendors: MTU sizing commands don’t match up.

A coworker was having a hard time getting OSPF to come up between a cisco and juniper router, I was going to do a full post on it, but this guy has done it much better than I was going to: http://thenetworksherpa.com/ospf-master-the-mtu-madness/

Basically, it comes down to this: when you change the hardware mtu on cisco stuff IOS automagically configures the IP mtu to the hardware mtu + 24 bytes (18bytes for the ethernet header plus some pad for layer 2 or other goodies).  Juniper doesn’t make that change for you.  So if you change the hardware mtu, you need to set the IP mtu up as well.