Router with 2 (or more) interfaces in same network

Crist Clark crist.clark at
Tue Nov 11 17:38:23 UTC 2003

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.  Other routers (of
> note FreeBSD boxes) do this just fine.

Errr, no. FreeBSD won't let you do this.

  # ifconfig fxp0 inet
  # ifconfig ep0 inet
  ifconfig: ioctl (SIOCAIFADDR): File exists

The error is a round-about way for the system to tell you, "hey, genius,
I've already got a route for that network."

You _used_ to be able to do this (oh, over two years ago?). The address
was assigned to the interface, and the error from trying to add a duplicate
route was simply ignored, no route got added anywhere. You can figure out
when the change was made by examining the code or by seeing when the 
maillists started to get flooded by people who could no longer do,

  # ifconfig if0 inet
  # ifconfig if0 alias

When they meant,

  # ifconfig if0 inet
  # ifconfig if0 alias netmask 0xffffffff

But to reiterate the problem here, it's not really assigning addresses
to interfaces, but trying to assign a route to the same network to different
Crist J. Clark                               crist.clark at
Globalstar Communications                                (408) 933-4387

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