SMTP Problems

[davepartridge]

Just purchsed 2 SB107 routers for a project, I have sonfigured them and they are working fine except that I can't send any email from OE larger than a few charecters, I can send very small emails no problem but anything bigger than a couple of lines fails. It was suggested that I lower the MTU size which I did but this made no difference. Does anyone have any ideas?

Also on another note I need to open 2 ports for PC Anywhere and wonder what access-list changes I need to make in addition to a port redirection. Not sure how to do the redirection.

Cheers

Dave

[davepartridge]

I have been scouring the internet for information about this SMTP problem and I can't find a way of resolving it. I have spoken to Cisco and even though I have just bought these routers they wont talk to me. Has anyone any information that may help please? I have resolved the other issue of the forwarding without too much digging.

Thanks

[Richard Burts · Accepted solution]

Dave

It might be helpful if you would supply a few more details about how these routers are connected and how they are configured.

I have seen symptoms similar to what you describe that do turn out to be issues with MTU, especially when the routers are connected by GRE tunnels, or by IPSec VPN, or something that encapsulates somewhere along the path. Depending on how you changed the MTU it may not have solved the problem. There are a couple of approaches to this and the one that I find the most simple is to adjust the mss of sessions from the end stations. I suggest that you configure this command on the LAN inteface where the end stations are connected on both routers (in interface configuration mode):

ip tcp adjust-mss 1350

Give it a try and let us know if it helps your problem.

HTH

Rick

[davepartridge]

Rick you are indeed a star ;o)))))

This fixed it instantly when I applied it to the ADSL Dialer0 Interface, just for my own piece of mind can you tell me what this does and how it affects the SMTP traffic?

Be lost without you m8 cheers

[Richard Burts]

Dave

Thanks for the kind words. I am glad that I was able to help you solve your problem.

Here is the brief explanation: when end stations establish a TCP session they negotiate an MTU that is related to the mss (maximum segment size). Normally that negotiation setablishes an MTU that matches the interface. However if there is something in the middle like GRE or IPSec VPN it may introduce the need to fragment. But the TCP packets may have the Dont Fragment bit set. If DF is set and the packet needs fragmentation, then the packet must be discarded. The ip tcp adjust-mss command tells the router that as it sees the packets go by that negotiate the TCP session to re-write the mss in the packet to a smaller size. The end station now negotiates a max size that will not require fragmentation.

So in your case the end stations will negotiate a TCP session to exchange email. The natural negotiation produces a max size that does require fragmentation which was breaking email if the messages were very large. Now this configuration limits the max size that they will negotiate so that fragmentation is not required.

HTH

Rick