Router with 2 (or more) interfaces in same network

Richard A Steenbergen ras at
Tue Nov 11 15:34:31 UTC 2003

On Tue, Nov 11, 2003 at 09:55:34AM -0500, Leo Bicknell wrote:
> In a message written on Tue, Nov 11, 2003 at 08:35:34AM +0000, Sugar, Sylvia wrote:
> > I am curious to know if its possible to have a router with its two interfaces, say configured as, 
> > and Theoretically, i see nothing which can stop a router from doing this.
> Cisco's don't let you do this.  I have always considered that broken,
> although I'm sure Cisco thinks it's a feature.

I'm not sure how Cisco is wrong on this one. If you want 2 router
interfaces to have the same route and you actually want both of them to
work, it means at the very least you must have a non point-to-point
medium, such as Ethernet. In this case, the correct configuration would be 
a bridge-group and IRB, creating a virtual routed interface with 2 
physical ports for bridging.

> Other routers (of note FreeBSD boxes) do this just fine.  In almost all
> cases I've seen it done it was for more bandwidth to the box (typically
> inbound only, because there are no good tools on Unix boxes to split the
> traffic between the outgoing interfaces).

I love FreeBSD, but it's routing code is probably the thing you least want
to look to for examples on how things should be. BTW there is a netgraph
module for L2 hash-based load balancing (aka etherchannel without the
PAgP/LACP), but yeah the lack of ECMP and a reasonable switching method to
support it falls into the category of the previous sentence. :)

Richard A Steenbergen <ras at>
GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC)

More information about the NANOG mailing list