[c-nsp] ip tcp adjust-mss

Richard Clayton sledge121 at gmail.com
Tue Feb 12 08:04:06 EST 2013


I needed to use this command the other day, I have an 887VA-M and the BT
FTTC product, I bypassed the BT modem and connected directly into the BT
wall socket with the 887VA-M as it has a VDSL interface (just a config

The config I was using was PPPOE which adds 8 bytes to the frame so on my
dialer0 I set the mtu to 1492, without the 'ip tcp adjust-mss 1452' I could
not open web pages or run speed tests as my local host's mtu defaulted to
their local setting (1500) and was dropped.

After adding the command to the LAN interface the router intercepts the SYN
packet from the local host and changes the maximum segment size to the
value stated before passing on to the remote host, the TCP 3 way handshake
is then completed with the two hosts agreeing on the lowest of their 2
values, obviously it doesnt help if you have large UDP packets but there
shouldn't be too many of those around anyway.

Using this command reduces the mtu size for TCP traffic flowing through the
configured router but in your case I would be more interested in why you
think you need it and where, do you think you have mtu bottlenecks in your
network that are causing fragmentation and if so can you just fix those
areas rather than adding this to lots of other routers.


On 11 February 2013 19:56, Eric A Louie <elouie at yahoo.com> wrote:

> I just put in this command on my upstream interfaces to help my mpls
> network
> pass traffic - that is, my effort to eliminate fragmentation in my
> backbone.
> Is anyone else using this method of "mtu control"?  I need some support -
> my CEO
> is asking why I have to do this, and who else does it, and is it a common
> practice, etc, so I'm looking for evidence, more than just "The Cisco TAC
> told
> me to do it".
> thanks
>  Much appreciated, Eric
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