GRE Tunneling and MS Exchange

[vincent-n]

Here's a scenario, both router A and B are connected via an MPLS cloud. both routers and MPLS communicate with one another via RIP. I've configured GRE tunnel between router A and B and pass EIGRP over this tunnel. Here is the problem:

- ALL applications (telnet, citrix, dns etc) are running fine except for Outlook when connecting to an Active Directory MS Exchange server.

When I turn off the GRE tunnel (hence EIGRP also), Outlook will start working again.

Thanks for your help.

[Georg Pauwen · Accepted solution]

Hello Vincent,

when GRE is configured, it is possible that you would need to adjust the mss value on your routers. Try and configure the following on the LAN interfaces of both your routers:

ip tcp adjust-mss 1350

and see if that makes a difference...

Regards,

GP

[vincent-n]

Thanks for your reply. The suggestion works flawlessly. I did try another (longer) solution and my one also works. It's to do with turning off the DF bit which was generated by MS Exchange2K3 (but not by Exchange 5.5). I created an ip extended ACL (101) that matches both the source and destination IP subnets, create a route map with a match statement pointing to the ACL and do a set ip df to 0.

[klorenzo]

Thanks for the 2 suggestions. We're using Exchange 2000 and encountered the same problem until I changed the mss on our gre tunnel then owa and outlook started working. ]

Great solution.