[c-nsp] ip tcp adjust-mss

Tim Franklin tim at pelican.org
Thu Feb 14 05:53:45 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
> tweek)
> 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.

You really shouldn't have to do this.  BT 21CN supports baby-jumbos to give you the extra 8 bytes of PPPoE on top of a 1500-byte IP packet.

Raising MTU on the physical Ethernet interface facing the BT modem definitely works, I can get 1500-byte DF-set IP packets to and from the Internet; you should be able to do the same on the VDSL interface.


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