Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!ncar!csn!boulder!daemon
From: P.Elford@aarnet.edu.au (Peter Elford)
Newsgroups: comp.dcom.sys.cisco
Subject: Re:  Routing query
Message-ID: <34995@boulder.Colorado.EDU>
Date: 13 May 91 04:24:28 GMT
Sender: daemon@boulder.Colorado.EDU
Lines: 90


On Thu Apr 18 10:54:17 1991 I wrote:

>...
>
>We are about to install two E1 circuits as shown below:
>
>                                ACT                      NATIONAL
>                          +---------------+   2Mb   +---------------+
>       NSW                | 139.130.188.2 +---------+ 139.130.188.1 |
>+---------------+   2Mb   |               |         |               |
>| 139.130.192.2 +---------+ 139.130.192.1 |         |               |
>|               |   48K   +---------------+         |               |
>| 139.130.72.2  +-----------------------------------+ 139.130.72.1  |
>+---------------+                                   +---------------+
>
>
>The 48K between ACT and the national hub will be retired when the 2Mb lines
>go in, but the 48K between NSW and the national hub will be retained for
>backup.
>
>HOW DO WE SET UP OUR ROUTING TO DO DYNAMIC FAILOVER TO THE 48K IF ONE
>OF THE 2Mb LINKS DIES ?

This is a summary of the responses I got and our eventual solution; many
thanks to those that answered.

>My current thinking goes as follows:
>
>A) Stick with RIP. We can only do this if there is some method of
>artificially increasing the hop count of routes sent from NSW to
>the national hub over the 48K line. The "offset-list" parameter only
>allows you to fiddle routing metrics on a per network, rather than 
>per interface, basis.
>
>IS THIS TRUE ? CAN WHAT I WANT BE DONE ?

It cannot be done using RIP metrics. It is possible to do it using 
cisco's administrative weight "meta-metric":

The NSW hub assigns an administrative weight of 254 to routes coming
from 139.130.72.1 (the national hub). The national hub assigns an
administrative weight of 254 to routes coming from 139.130.72.2 (the
NSW hub).

Since 254 is the largest possible value (about from 255 which says
never believe), neither NSW nor the NATIONAL hub will believe the
routes the they receive from each other, unless they are the *only*
routes they receive (which is what happens if one of the 2Mb lines goes
down).

It takes about 3-5 minutes for the failover to the 48K to work because
of the various timers within RIP, but it recovers almost instantly.

>B) Fire up IGRP to be able to dynamically detect the failure of one of
>the 2Mb lines. NSW and ACT routes will have to distributed between all
>three routers but since most of them have are learnt via RIP, to
>redistribute them using IGRP we will have to use a "default-metric"
>for them all.
>
>DOES THIS MEAN THAT ALL RIP ROUTES REDISTRIBUTED BY IGRP WILL HAVE THE
>SAME METRIC REGARDLESS OF WHICH INTERFACE THEY GO OUT OVER ?

Yes. Quite a few respondents said we should just IGRP, but for such a
simple topology and a simple requirement this seemed (and was) unnecessary.

>ARE THERE ANY OTHER WAYS OF PASSING DEFAULT GATEWAY INFORMATION AROUND ?

Lots! We settled on a default network solution like this at the NATIONAL
router

router rip
redistribute static
default-network 132.160.0.0
!
ip route 132.160.0.0 139.130.x.y

The ACT and NSW routers then just have default-network 132.160 and failover
works the same as for any other network.

>DOES AN IGRP "EXTERIOR ROUTE" OVERRIDE A RIP DEFAULT NETWORK (0.0.0.0) ?

Yes.

Peter Elford,                           	e-mail: P.Elford@aarnet.edu.au
Network Co-ordinator,	 			phone: +61 6 249 3542
Australian Academic Research Network,		fax:   +61 6 247 3425
c/o, Computer Services Centre,			pager: +61 6 245 3035
Australian National University			post:	PO Box 4     
Canberra, AUSTRALIA					Canberra 2601