Subject: Re: Non-BGP multihoming
To: Ken Hornstein <>
From: Andreas Wrede <>
List: current-users
Date: 10/10/1997 10:28:50
On Fri, 10 Oct 1997 09:59:59 EDT Ken Hornstein writes
> >Paul Vixie published patches for BSD/OS 2.1 which allow for a default
> >route per interface and which remember the interface via which a tcp
> >connection was initiated. See
> >for more.
> Okay, I guess I'm just stupid this morning ... what are the gains in
> using this?  I have a multihomed machine at home, and I don't see how
> doing this would gain me anything.
> I'm sure there _is_ a gain ... I'd just like to understand it :-)

If a distant host will tcp-connect to your multi-homed machine via the 
interface which does not have the default route then your traffic back to 
that machine will be routed asymmetrically, ie it will leave through an 
interface different from the one it came in on. That means that if the 
machine is a web or ftp server, the interface with the default route will 
carry all outbound traffic, the other interface will carry none. 

Note that the underlying assumption is that the you are connected to two (or 
more) difference Autonomous Systems, ie. incoming traffic will actually take 
widely different routes.

Andreas Wrede              Planix, Inc.         Networking, System Administration, Consulting      Toronto, Ontario, Canada