Should I be using "ip tcp adjust-mss" on VPN router?

Unanswered Question
Nov 20th, 2007

Hi, I have never set "ip tcp adjust-mss" on any of my VPN DSL routers, should I be setting this? Many example I see use this, what does it do?

I have this problem too.
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whiteford Tue, 11/20/2007 - 09:35

Thanks, for some reason I can't open that link on my blackberry, does it show a command to see if I'm getting fragmentation, if so do I set the mtu on the dialer and ethernet 0 router?

Edison Ortiz Tue, 11/20/2007 - 09:46

Fragmentation can be seen from the end device being affected by taking a packet trace.

It's recommended to place this command on the interface that is fragmenting the packet, often that is the tunnel interface due to GRE.

whiteford Tue, 11/20/2007 - 10:04

The end point is a Cisco concentrator0and on the filtered events I do are fragmentat errors every now and then, shoul I set something?

Also I don't set the mtu on the DSL dialer interface, does this default to 1500? How do I know if I should lower this?

Edison Ortiz Tue, 11/20/2007 - 10:49

If you are seeing fragment errors in the Cisco concentrator then configuring "ip tcp adjust-mss" will help.

Keep in mind, the commands relies on ICMP. Therefore, any ACL on the path back to the affected device must be observed and allow ICMP.

As for the DSL interface, you can find out its default IP MTU by typing show ip interface dialer x

whiteford Tue, 11/20/2007 - 11:20

I haven't set the mtu, should I? Its currently defaulting to 1500.

I see some sample configs set the mtu to a lower value.

Edison Ortiz Tue, 11/20/2007 - 18:13

leave the MTU as it is for now and just implement the "ip tcp adjust-mss" feature.


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