MTU and adjust-mss for VDSL PPPoE termination

[Mike D]

Hi guys.
I have a couple of questions regarding the MTU and adjust-mss config when terminating PPPoE/VDSL on a router.

1. Firstly I guess there are 2 scenarios I would like to discuss;

A - Both VDSL and PPPoE terminate on the Cisco router
B - Cisco router is a PPPoE client only with the telco modem used for VDSL termination.

In normal conditions I would add "ip mtu 1492" and "ip tcp adjust-mss 1452" to the dialer interface only.

This usually seems to work okay, however after reading many other posts on the topic there seems to be some dispute as to alternatively applying the MTU and adjust-mss config to the inside (LAN) facing interface.

Is there a standard for this in both scenarios? Both in terms of VDSL and PPPoE on the same Cisco device (eg 887VA) and in terms the Cisco device being a PPPoE client only?

Would the config specific to MTU and adjust-mss be the same or different in each scenario?

2. Secondary if ADSL is instead used (so ADSL/ATM as opposed to VDSL/Ethernet) then how does this affect these same settings?

I know this topic, or similar, has been discussed before, but there appear to be many different views out there.

I hope you can help!

Many thanks.

Mike

[Joseph W. Doherty]

This usually seems to work okay, however after reading many other posts on the topic there seems to be some dispute as to alternatively applying the MTU and adjust-mss config to the inside (LAN) facing interface.

The adjust-mss command should work anywhere along the path, for traffic flowing in either direction.  The reason you would want to avoid applying on an internal facing LAN interface, in case that interface also transports traffic that doesn't need the adjustment.

As the adjust-mss only impacts TCP traffic (and TCP traffic whose initial handshake is seen by the command), MTU is used to set MTU to what's actually supported.  Unlike adjust-mss, MTU only impacts egress traffic.  Similar to adjust, you wouldn't want to apply to a LAN facing interface as it could impact traffic to which it doesn't apply.  Also, inbound (WAN) traffic, should have already been fragmented before the LAN interface "sees" it, making placing it there, moot.  Ideally, the MTU command would be set on the first egress hop to which has the lower MTU is in effect.