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List:       inet-access
Subject:    Re: Duplicate IPs on channelized T1s? (Cisco)
From:       "Erick B." <erickbe () yahoo ! com>
Date:       2001-10-29 1:31:25
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Hi Nicholas,

Cisco IOS lets you put the same IP network on upto 4
Serial/WAN interfaces. Thats as much as I can tell
you. 

I have seen this used for primary and backup links.
This lets them use same IP subnet on both. It may also
be a old way of doing load-balancing between sites or
saving on IP address space.

I did play around with this a few months back just for
kicks. I setup 2 routers back to back that had
multiple interfaces connected and put same IP network
on each with a RIP or IGRP and I got multiple equal
cost routes. I then added more routers so it wasn't
back to back, more like a hub and spoke setup and
things weren't pretty and it didn't work well. I
didn't think it would, but just was playing around. 

Hope this was helpful. I'd be interested in more
details on this as well.

--- Nicholas Bastin <nbastin@opnet.com> wrote:
> Having seen this in a number of configs, and having
> gotten conflicting
> answers from the TAC and people who actually have
> used the hardware, I
> thought I'd ask this here.
> 
> I've seen a number of channelized interfaces on the
> same controller (say
> channel groups 0 and 2 on the same T1 controller)
> that have the same IP
> address (and are not administratively shutdown).  My
> question is, is
> this legal (and if it is, what does it do?)  I don't
> have any gear with
> the DSUs built into the interface cards, so I can't
> test this myself. 
> Normally, I'd treat it like any other interface, and
> obviously this
> wouldn't really work, but an engineer at the TAC
> sent me an email
> indicating that this was a legal config (he didn't
> say what the router
> did in that case).  My guess is that he didn't
> really understand the
> question, but I thought I'd ask here to see if I
> could get a better
> response (is it just me, or has the quality of your
> average TAC engineer
> seriously degenerated over the last couple of
> years?)
> 
> --
> Nick
> 
> 
> -
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