pls help about mtu setting

Tom Pipes tom.pipes at
Thu Jun 17 13:39:29 UTC 2010

Are you authenticating with PPPoE?  If so, it has a maximum MTU size of 1492 due to the encapsulation overhead. 

Tom Pipes 
T6 Broadband/ 
Essex Telcom Inc 
tom.pipes at 

----- Original Message ----- 
From: "William Herrin" <bill at> 
To: "Deric Kwok" <deric.kwok2000 at> 
Cc: "nanog list" <nanog at> 
Sent: Thursday, June 17, 2010 8:36:00 AM 
Subject: Re: pls help about mtu setting 

On Thu, Jun 17, 2010 at 8:19 AM, Deric Kwok <deric.kwok2000 at> wrote: 
> My DSL company asks me to set the modem 146 2 and my old company used 14 92 
> Why it is not standard 1 500? 

Because they're wrapping your packet inside another packet that they 
then transmit on a line with a 1500 byte MTU. The header on their 
packet needs a few bytes, so you can't have them. 

Bill Herrin 

William D. Herrin ................ herrin at  bill at 
3005 Crane Dr. ...................... Web: <> 
Falls Church, VA 22042-3004 

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