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Community Member

OSPF Neighbor stuck in Exstart


We have an existing network with OSPF configured between 3750's (DR and BDR) this is working fine -

But the Nexus 7k which has been configured to run OSPF, won't established the adj. It is in the same network/subnet.

The MTU is 1500.

stuck in EXSTART

Thanks for the help

Everyone's tags (3)
VIP Super Bronze

OSPF Neighbor stuck in Exstart


Can you add mtu-ignor under the interface and try again?


Community Member

OSPF Neighbor stuck in Exstart

Stevens, found this article for EXSTART issue -
hope this helps you

Community Member

Re: OSPF Neighbor stuck in Exstart

So we have done some testing and it seems to be the vPC in the middle of the design.

So OSPF between 3750A and 3750B is ok, but the neighbours do not come up between OSPF on Nexus VSS and the 3750's

Any suggestions please

OSPF Neighbor stuck in Exstart

Your design will not work unfortunately as it will result in routed traffic from the VSS pair to the Nexus 7000's to traverse the vPC peer link. Under normal conditions (where there has been no vPC port or link failures) the traffic crossing the vPC link will be dropped because of the Nexus' in built loop prevention logic.

The correct design is to form an OSPF adjacency from the VSS to each Nexus individually, and then another between the Nexus' over the vPC peer link.


OSPF Neighbor stuck in Exstart

Hey Dominic,

Are you using a Layer 2 or Layer 3 vPC? << if L2 / L3 dont make problems for your design, try:

1. if Layer 2, Make sure you configure RPVST+ on all you devices.

2. if Layer 3, Make sure you configured OSPF to use IETF NSF on your Cat 6500 VSS.

check your Interface MTU and IP MTU... double check the OSPF timers to be identical on your neighors.

** system MTU change Needs a reboot **

hope it helps!

Hope it Helps!

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