OSPF down due to too many retransmissions

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Jan 27th, 2009
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Hi All,

We have around 5 routers in a single LAN where we are running OSPF. The issue is one router we configured the MTU as 1546 bytes but the other routers are configured with 1500 bytes. Initially we had OSPF neighborship issues on the LAN and were getting the logs "OSPF down due to too many retranmissions".

While checking found that the MTU mismatch is the cause of this.

Option is either to configure the same MTU size on all the routers or to configure "ip ospf mtu-ignore". Since we were not able to configure the same MTU we went and configured "ip ospf mtu-ignore" on all the routers post which everything started working fine.

But still we are facing the OSPF neighborship issue couple of times and it comes up automatically. Still getting the same OSPF down due to too many retranmission:"

I would like to know the only solution for the above issue is to have the same MTU size on all the routers?


Arun Kumar

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Giuseppe Larosa Tue, 01/27/2009 - 07:10
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Hello Arun,

the best solution is to have the same ip mtu on all devices connected to the same LAN segment.

to be noted that at interface level you can:

increase the generic MTU with

mtu 1546

keep a standard 1500 byte mtu for IP with

ip mtu 1500

add other specific per protocol mtu commands for example for MPLS

mpls mtu 1542

another suggestion is to make the router with different settings DR other on the segment so that its activity is reduced to minimum.

For doing this you can apply

ip ospf priority 0

in interface configuration mode

Hope to help



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