VPN Authentication Issue with 871

verbbbie2 · ‎05-17-2010

All

I have an Cisco 871 at a remote site and at the head end I have a Checkpoint Firewall; I can get the tunnel to come up and everything seems to work okay except for Outlook to Exchange server and File Mappings basically anything using AD Authentication. I took out the 871 and replaced it with a PIX501 and everything works perfectly. I also tried 2 different 871's with the same results so I am guessing I am missing some type of command in the 871 to fix this but I can't seem to figure out what it is. Anyone else ever have this problem and can you share the resolution please? I have attached my sample configs of the PIX and 871.

Jennifer Halim · ‎05-17-2010

Sounds like an MTU issue.

Add the following on the router internal interface:

interface Vlan1

ip tcp adjust-mss 1380

Hope that helps.

verbbbie2 · ‎05-17-2010

Perfect seems to be working; can you explain why?

Jennifer Halim · ‎05-17-2010

PIX firewall by default has the following configuration "sysopt connection tcpmss 1380" hence during the TCP three way handshare negotiation for the MSS size, it negotiates for a lower MTU size.

MTU size gets larger after it is being encrypted (with the ESP header) and ethernet normally has 1500 MTU size. If the MTU gets larger than 1500 bytes, the packet gets fragmented which causes issue that you are seeing.

Check out the "sysopt connection tcpmss" command explaination here:

http://www.cisco.com/en/US/docs/security/pix/pix63/command/reference/s.html#wp1026942

Hope that answers your question.