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New Member

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

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?

7 REPLIES
Hall of Fame Super Bronze

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

Yes, it's recommended to apply such command if you are experiencing fragmentation issues.

Please see this URL for detailed information on such command:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/hbba_r/bb_a1h.htm#wp1121272

New Member

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

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?

Hall of Fame Super Bronze

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

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.

New Member

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

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?

Hall of Fame Super Bronze

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

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

New Member

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

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.

Hall of Fame Super Bronze

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

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

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