Changing MTU to resolve an IPSEC tunnel that won't establish?

[CiscoPurpleBelt]

I know many times you must change the MTU to less than the default otherwise certain Aps, sites, etc. the traverse the tunnel have poor performance, but are there instances where MTU could play a part in just establishing Ikev1 tunnels between two routers? 

Don't have the debugs right (to verify where it is failing or what errors seen) here but all parameters are the same on both ends, keys, etc., IPSEC negotiation traffic makes it to both ends, wondering if MTU could play a part just on tunnel negotiations.

[Deepak Kumar · Accepted solution]

Hi,

I have never seen that MTU could cause of negotiation issue. Yes, I faced slowness or some of the services not working issue due to MTU. I would like to check logs and configuration.

[Richard Burts · Accepted solution]

I can not think of a circumstance where MTU would prevent ISAKMP negotiation. It really would be helpful to see the debug output.

[CiscoPurpleBelt]

Yes that is what I thought, had to make sure.

Too challenging to post on here, I can't even get to the remote site in any fashion LOL.

Thanks again!