From: Olive, Darren (Darren.Olive@globalcrossing.com)
Date: Thu Sep 05 2002 - 05:13:29 GMT-3
Claudine
If you mutually redistribute between multiple RP's with no filtering, the
possibility arises for routes to be re-injected back into the source domain.
Split-horizon is only concerned about not advertising a route out of an
interface that it was originally learned on within the routing process.
Redistribution is an internal process within the router & without filtering,
the router has no method for deciding what it should & should not
redistribute.
As an example:
If we have two routing domains running RIP & IGRP which are mutually
redistributed on a single router. The redistributing router populates the
route table with a RIP learned route (AD 120) for a network in the RIP
domain. This route is redistributed into the IGRP domain & subsequently an
update for this network arrives back on the redistributing router. The
router populates the route table with this route as an IGRP learned route
(with a better AD of 100) & redistributes it into the RIP domain. The RIP
routers now see the redistributing router (& subsequently the IGRP domain)
as the next hop for the original RIP network. All traffic will be forwarded
to the redistributing router & into the IGRP domain from where the traffic
will loop until the TTL kills the packet.
I understand that this is not very clear without a diagram, but hope it
helps all the same.
Thanks
Darren
-----Original Message-----
From: Claudine DEMAR [mailto:lkcnet00@hotmail.com]
Sent: 04 September 2002 16:14
To: Olive, Darren; ccielab@groupstudy.com
Cc: ccie@cisco.com
Subject: RE: redistribution
Darren,
Thanks for your answer. I'm not sure of what you mentionned about multiple
RPs redistributing on the same router. I thought "split-horizon" (or what
the Cisco book TCP/IP Routing&Switching calls so) would prevent this to
happen, regardless of the distance of the RPs.
RP1, RP2, RP3 running on the same router and provided that the different RPs
do not share any prefixes, a RP1 would never redistribute to RP2 routes that
it would have learnt from RP3. Please correct me if I'm wrong.
Thanks,
Claudine
>From: "Olive, Darren" <Darren.Olive@globalcrossing.com>
>Reply-To: "Olive, Darren" <Darren.Olive@globalcrossing.com>
>To: "'Claudine DEMAR'" <lkcnet00@hotmail.com>, ccielab@groupstudy.com
>CC: ccie@cisco.com
>Subject: RE: redistribution
>Date: Wed, 4 Sep 2002 10:00:27 -0400
>
>Claudine
>
>By 'one-point-redistribution' I assume that you mean redistribution between
>only two routing protocols on a single router. If multiple routing
>protocols
>& redistribution are in effect on a single router, the possibility arises
>for routes to be re-injected back into the source routing domain because of
>the various administrative distances of the routing protocols involved. I
>would be inclined to configure filtering to allow the routing updates to
>travel the one required direction only.
>
>Thanks
>Darren
>
> -----Original Message-----
>From: Claudine DEMAR [mailto:lkcnet00@hotmail.com]
>Sent: 04 September 2002 13:59
>To: ccielab@groupstudy.com
>Cc: ccie@cisco.com
>Subject: redistribution
>
>Hi.
>
>I would like to know whether CCIE candidates are expected to configure
>route-maps to filter redistribution between routing protocols even if we
>are
>
>in a one-point-redistribution scenario (there's only one common router
>between the different routing domains).
>
>Should those route-maps be configured systematically even if there's no
>need
>
>for them and even if there's no such mention in the lab instructions ??
>
>Thanks,
>
>Claudine
>
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