Subject: Multi-topology interest

From: "CAUCHIE Gregory RD-CORE-ISS" <[email-masked]>
Date: Thu, 26 Jul 2007 16:09:09 +0200

Hi all,
 
as opinions were requested tuesday during the rtgwg meeting regarding
interests on IGP muti-topology applicability, here is mine.
 
Today, most of multicast services are deployed using the PIM protocol.
With PIM, multicast trees are set up by relying on IGP informations,
thus decisions to send join and prune messages are made upon an IGP
adjacencies status basis. But for multiple reasons, the IGP adjacencies
states may not exactly overlap the PIM adjacencies states. In that case,
the shortest path from one router to a multicast source or a RP can
include a link where at least one PIM interface is down. The consequence
is that PIM join messages are sent on a dead end and then multicast
service is broken until some other actions (that may request human
direct actions) are made.
 
To prevent this issue to occur and thus unavailability of multicast
services (like IPTV), multi-topology is a real interesting solution.
Multi-topology could be use to convey in LSPs or LSAs information of
links where a PIM adjacency is UP. Thus LSDB would then have a "PIM
topology" in parallel of a classical network IGP topology so that the
former could be use as a basis for SPF computation from the router to
the multicast source instead of the latter. The PIM join and prune
messages would then be sent on the best "multicast available" way if it
exists.
 
This is to me the first interest of multi-topology work.
 
Greg
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