Cisco Tips & Tricks

October 18, 2009

System MTU / ip ospf mtu-igonore

Filed under: ccie, IP Routing, ospf, Switching — ciscotips @ 6:54 pm

I was working on OSPF lab and suddenly on my 3560’s I saw a OSPF adjacency errors.

%OSPF-5-ADJCHG: Process 1, Nbr 192.168.10.1 on
FastEthernet0/0 from EXSTART to DOWN, Neighbor Down: Too many
retransmissions

%OSPF-5-ADJCHG: Process 1, Nbr 192.168.10.1 on
FastEthernet0/0 from DOWN to DOWN, Neighbor Down: Ignore timer expired

Suddenly I realized that may be I have MTU issues as I just completed a Q-in Q lab and changed my switch MTU ,  but to double check

I thought of checking a “debug ip ospf events” … and here it was.

OSPF: Rcv DBD from 192.168.10.1 on
FastEthernet0/0 seq 0x12A6 opt 0x52 flag 0x7 len 32  mtu 1504 state
EXSTART

OSPF: Nbr 192.168.10.1 has larger interface MTU

There are multiple ways to fix this, you can either issue “system mtu 1500” on switches or use an interface level command on Routers “Ip ospf mtu-ignore” . or the third one will be to change MTU on router interfaces ( Least preferred).

when value is changed, it will not be stored in neither running-config
nor startup-config. On Catalyst 3550, this information is stored in a
separate file on the flash. On Catalyst 3560, you can’t see it, unless
you do “show system mtu”.

“system mtu 1500” on switches is the default command. Even when value is changed, it will not be stored in  running-config  or startup-config. On Catalyst 3550, this information is stored in a separate file on the flash. On Catalyst 3560, you can’t see it, unless you do “show system mtu”.

This is one of the well-know gotchas on the actual lab exam.You have to know how to solve this. Hence, when configuring routing protocols on switches, make sure you know what the MTU is.

One important thing to note is that you might break stuff in the lab if you were suppose to configure MTU for q-in q lab and later you changed “System MTU” in your switch to fix OSPF issue.

You can also use system mtu routing 1500 in your switch to fix OSPF issue as this will be only used for routing but for Q-in-Q lab switch MTU will be still used as 1504.

IMHO,Best way to fix this in the lab would be “ip ospf mtu-ignore” under the interface on your router.

MTU  has to be the same on both ends of the link before the neighbor can form adjacency.

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8 Comments »

  1. Only the side with the lower MTU does not form the OSPF neighbor. If the router or switch has a higher MTU, they are more than willing to form a neighbor relationship.

    That is why the ‘ip ospf mtu-ignore’ usually only needs to appear on the router side; because the switch has the higher mtu.

    Comment by Bhavesh — October 29, 2009 @ 1:58 pm

  2. Switch doesn’t have a higher MTU by default. As I said earlier if you have changed the default MTU to support Q in Q configs. you will have issues forming OSPF adjacency.

    HTH

    Comment by ciscotips — October 29, 2009 @ 2:30 pm

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