[c-nsp] MTU size difference and OSPF

[Jason Koh]

Hi all

I encountered this problem when configuring a Cisco 7206 NPE-G1 as a PE for MPLS. This router is linked to a Cat6509 with its MSFC 2 running OSPF with the 7206. IOS version for the 7206 is c7200-k91p-mz.122-28.SB2.bin

The problem arises when I have to fit the GigaEthernet interface of the 7206 to accomodate packets with bigger than 1500 due to MPLS. On the main interface,
I have to use this command

#interface Gi0/1
#mtu 1520

Without this command, I cannot key in the following commands in Gi0/1.1

#mpls mtu 1520

This allows me to send packets with the MPLS labels as well as keep the normal IP MTU to 1500.

However at this point, if the OSPF between the Cat6509 and the 7206 is broken for any reason, it will never reestablish. It will always loop between EXSTART/DOWN because of the MTU difference. 

Dec  4 15:25:19: %OSPF-5-ADJCHG: Process 10, Nbr xxx on Vlan11 from FULL to DOWN, Neighbor Down: Dead timer expired
Dec  4 15:29:52: %OSPF-5-ADJCHG: Process 10, Nbr xxx on Vlan11 from EXSTART to DOWN, Neighbor Down: Too many DBD retransmitions
Dec  4 15:30:37: %OSPF-5-ADJCHG: Process 10, Nbr xxx on Vlan11 from DOWN to DOWN, Neighbor Down: Dead timer expired
Dec  4 15:31:52: %OSPF-5-ADJCHG: Process 10, Nbr xxx on Vlan11 from EXSTART to DOWN, Neighbor Down: Too many DBD retransmitions

I have put in ip ospf mtu-ignore on the 7206 but it still can't come up. 

Anybody encountered something like this?


Jason Koh
Senior Network Engineer
Pacific Internet Ltd