Subject: [pim] PIM-SM multi-access link Assert Question

From: Matthew Hepburn <[email-masked]>
Date: Mon, 19 Sep 2005 10:49:05 -0700

Hi,

I am new to PIM-SM, but I believe there may be a small problem with  
asserts on edge networks.

On a multi-access network with 2 or more PIM-SM routers, and 1 or  
more hosts registered to a group shared between those routers there  
seems to exist a chance for deadlock.

The key to the bug is that no traffic is currently flowing down the  
trees and through the routers onto the multi-access link. DR election  
will be performed, but no asserts will ever be sent because of the  
lack of traffic. As I understand it all the downstream interfaces  
should be in the Assert No Info state, and only the DR will have  
CouldAssert = True.

The problem arises when the nexthop to the RPF is lost on the DR. The  
DR's CouldAssert will become False, but since the DR is not in the I  
Am Assert Winner state it will not send a canceling assert. Since the  
DR did not send the Canceling assert the other routers on the multi- 
access link will not be aware of it's failure and will remain unable  
to assert, since they are not the DR, and there are no PIM joins to  
make their CouldAssert's true.

Since the non-DR routers will not challenge the DR, traffic received  
via their RPF interfaces will not be forwarded onto the multi-access  
link, and the link will never receive any of the traffic from the  
multicast Tree.

In this case, would it not make sense for the DR to start with the  
assumption that it is the assert winner? Any input or clarification  
will be greatly appreciated.

  - Matthew

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From: Christopher Thomas Brown <[email-masked]>
Date: Mon, 19 Sep 2005 15:17:46 -0400

If the DR has lost the route to the RPF and given no PIM join state on
that network then no forwarding path will exist between the network and
the RP(G) for *,g state or S for S,G,spt state.  The ability of the
other routers to assert after this is irrelevant since no traffic will
reach them to trigger the assert mechanism.

Note: This is not properly a deadlock since the nondelivery of traffic
resolves itself once the DR relearns the route to the RP(G) or S.

Finally, this does not seem like much of a problem. What likelyhood do
you see for ONLY the DR on the network to lose its route for a
significant length of time?  If the other routers on the network retain
a route then the DR should quickly learn it from them unless you are
forcing things with policy, in which case, you probably have a lot of
other difficulties keeping the PIM domain connected and convex.

Chris

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From: Mark Handley <[email-masked]>
Date: Mon, 19 Sep 2005 19:16:22 +0100

This would only be a problem if the routers didn't talk to each other.
 If they did, then the DR should receive a route to the RP through one
of the other routers on the LAN, and will join via that router over
the LAN.

 - Mark