MTU vs tcp adjust-mss

[tato386]

My server is having trouble exchanging mail with another SMTP server. After much troubleshooting I ran accross an article that states that this might be caused by a "black-hole" router and MTU/packet issues. Sounds weird but I want to give it a try. My questions are:

1) Should I use "ip mtu xxx" or "ip tcp adjust-mss"? What are the diferences between these two commads.

2) Should I apply this to the WAN or LAN of my Internet router?

Thanks,

Diego

[mbroberson1]

Rick,

Great response! Before I read this post reply I was think about the networks beyond control, those with lower MTU values, differnt circuit (technologies) types and such.

This seems to be a fairly interesting topic to many. I guess that can be so when things are so clear in documentation or conflicting docs.

Thanks,

Brandon

[Mike_Gannon1]

By setting the "ip tcp adjust-mss" command on the interface you tell the router to update the MSS header in the TCP SYN packets that are part of the three way handshake for any traffic transiting the link.

On tunnel interfaces (GRE/IPSec) where fragmentation may be disabled this allows an intermediate router to advertise the restriction to endpoints and so prevent any fragmentation being required during transit.