email issue with 2 WAN lines (BGP and default)

Unanswered Question
May 17th, 2007

Our branches have 2 WAN lines. The main line is using BGP and we use default with admin distance of 220 facing to the DSL (we use the VPN for the backup). So the idea is when the BGP line drops/down, the traffic will go to the default route (DSL). However, everytime this happens, the branch emails (outlook) will not work. Others may be able to access and some can't. All the other applications are working fine.

Please tell me what should we do. Thanks.

I have this problem too.
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sundar.palaniappan Thu, 05/17/2007 - 10:18

The problem you are facing may be a result of MTU limitation on the DSL network. Configure the command 'ip tcp adjust-mss 1440' under the LAN facing interface and test your email connection.



lleopoldo Thu, 05/17/2007 - 10:23

Hi Sundar,

Where will i put that command? in the router interface or to the DSL cpe facing the LAN?

lleopoldo Thu, 05/17/2007 - 10:31

Hi Sundar,

Yeh, i got it. I tried in the router's interface. I will let you know if this works. Thank you.

Manoj Wadhwa Thu, 05/17/2007 - 10:40

Dear Sundar,

I've got a quite similar problem in one of my routers. A site is using MPLS link. All applications are working fine except users complain that even for a 300 Kb mail, outlook hangs. We have done extensive tests on this but with no luck. Can you plz tell me what is the packet size when we send e-mail. Because, even if i do extended ping with source as my fast ethernet and destination as my exchange with 1500 bytes as datagram size, i dont see any drops. Any suggestions of this is highly appreciated. Thanks!

- Manoj

lleopoldo Fri, 05/18/2007 - 07:19

Hi, i just check and conduct an intrusive observation on this problem. When the main MPLS line goes down, the backup (default route) line via VPN assumes. Everything works fine then including emails (outlook). However, when main WAN line back up, that's the time we are getting problem. Some of the workstations can't access email anymore but everything is ok. Please help how we resolve this.


sundar.palaniappan Fri, 05/18/2007 - 15:42


There isn't enough information available to troubleshoot the problem. Do the workstations have ping reachability to the SMTP server when the WAN link comes backup. Where's the email server located at. Can you post the configuration and topology diagram?



sundar.palaniappan Fri, 05/18/2007 - 16:01


I don't know what the packet size(s) are for email communication. A sniffer should show the packet size and can help effectively troubleshoot the problem.

In general VPN and DSL/cable tend to create some problems with MTU because of overhead associated with the packets. To rule out MTU isn't an issue you can lower the MTU or use the command I had suggested in my previous post. If you are still having problems just with one application like email then use of sniffer can help isolating the problem if all other options are exhausted.




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