Mulicast and RPF

Unanswered Question
Apr 30th, 2009


I want to discuss a scenario.

lets say I have 2 paths to Mcast source or RP and I don't enable pim on the lower cost path,would my Mcast will work or it will not due to RPF failure.

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Overall Rating: 5 (1 ratings)
Giuseppe Larosa Thu, 04/30/2009 - 10:10

Hello Mohammad,

multicast packets should be dropped for RPF check failure.

You can have troubles also with equal path costs because unicast routing support them but RPF elects only one link as the best one (no parallel links support)

Hope to help


tahseen12 Thu, 04/30/2009 - 10:59

Hi Giuseppe

Ya with equal path costs it will select the RPF interface with higher ip address to the neigbor.

My concern was if we don;t enable pim on lower cost path and enable pim on higher cost path,it should not work due to RPF check.Just brainstorming:)Thats why pim shouild be enabled on all interfaces to avoid RPF check failures.

tahseen12 Thu, 04/30/2009 - 11:00

Moreover we can use tunnel to use both the links with ip mroute.

Giuseppe Larosa Thu, 04/30/2009 - 11:08

Hello Mohammad,

yes the GRE tunnel is the classic solution for two equal cost paths with pim not enabled on physical links.

In your original scenario it should be enough to enable pim on the unicast best path if possible.

The higher cost path will be used when the primary path fails

Hope to help


tahseen12 Thu, 04/30/2009 - 11:42

Hi Giuseppe

The higher cost path will be used when the primary path fails

only in case if i enable pim as well on higher cost path.

Harold Ritter Thu, 04/30/2009 - 20:46


Just to add that ss far as the equal cost past scenario is concerned, you could enable "ip multicast multipath" to get load splitting across these multiple paths.


Giuseppe Larosa Fri, 05/01/2009 - 00:46

Hello Harold,

thanks for this useful information.

I see this is not quite new and it is given in global configuration mode.

It is still a flow based load balancing using S,G or S,G and ip next-hop to decide which link to use.

We can say it emulates unicast CEF load balancing and allows to relax the RPF check on equal cost paths.

Best Regards



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