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Re: PPPOE, MTU, and boom.

  • From: Scott Silzer
  • Date: Wed Jul 18 04:01:52 2001

If you are using an 827 try the ip tcp adjust-mss command.

Taken from CCO:

Resolved Caveats for Release 12.2(2)XH

This section describes possibly unexpected behavior that are resolved in Release 12.2(2)XH.


The PPPoE standard sets the maximum transmission unit (MTU) of an Ethernet packet to 1492 bytes. If a host PC uses the default
MTU of 1500 bytes, the router between the server and the host would drop those packets. Even though an ICMP message is sent to
the website notifying it of the problem, some websites are programmed to ignore ICMP messages and would continue to send
1500-byte packets that will get dropped.

Starting with Release 12.2(2)XH, you can resolve this problem by changing the value of the maximum segment size (MSS) contained
in the MTU to 1492 or less. Use the following IOS command:

ip tcp adjust-mss mss

where mss is 1492 or less.

This command does not require NAT to be configured.

At 3:35 -0400 7/18/01, Alex Rubenstein wrote:
On Wed, 18 Jul 2001, Alex Rubenstein wrote:

 It allows you to tinker with the Windows settings of TCP. I adjusted MTU
 to 1400, wham-o, all the sites I listed started working; including, what I
 didn't list before, the downloading of transactions via Quicken. Seems to
 be a fixall.
I can tell it's getting late, when I reply to myself.

In the scenerio where you have a router be the PPPOE client (in my test
case, it's a Cisco 827 running 12.1(3)XG4), things are still somewhat
broken. The 827 is routing between an ethernet and a PPPOE session, and
even with the 827 having the MTUs at 1492 or 1400, windoze boxen are still
stuck up there at 1500 and those sites don't work. I then use Mr. TCP and
set the ethernet card of the windows box to a MTU of 1400, and voila, it

This really sucks for those folks who will have many machines bechind said
router; it will require a Mr. TCP on each and every one of them.


-- Alex Rubenstein, AR97, K2AHR,, latency, Al Reuben --
--    Net Access Corporation, 800-NET-ME-36,   --

Scott A Silzer

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