Subject: [pim] PIM-SM v2 deployment issues - 1 minute latency?

From: Philip Ho <[email-masked]>
Date: Thu, 01 Nov 2001 16:57:00 +0800

Hi all,

Considering the case of 1 group, 1 sender and 1 receiver in a multicast
network of PIM-SM v2. We are promoting IP multicast in Hong Kong and we
have created a mini-testbed involving 6 layer-3 switches running PIM-SM.
There are two questions that PIM-SM need to face during deployment.

1. Are there anybody out there notice that when sender starts before the
receiver. The receiver will have to wait 1 minute to receive data after
it joins the group.

2. We are afraid that the outgoing link at the router where the paths of
RPT and SPT diverge will be flooded with duplicate bandwidth of data.
However, after measurement, we cannot find any. Why? Am I correct to say
that it must join (S,G) and then receive data via SPT FIRST and send
(S,G) RP-bit Prune towards RP LATER?

Regards,
philip
Hong Kong IP Multicast Initiative (HKIPMI)
Department of Information Engineering
The Chinese University of Hong Kong
Phone : 2603 5240
Fax : 2603 5032

---

From: John Zwiebel <[email-masked]>
Date: Fri, 02 Nov 2001 09:18:36 -0800

> receiver. The receiver will have to wait 1 minute to receive data after

This is a bug in the implementation of the protocol in your switches.
Either the last-hop router is failing to trigger a (*,G) join on receipt
of the IGMP host report, or the RP is failing to convert that (*,G) 
join into an (S,G) join toward the source.  This is not a PIM protocol
problem. 
> (S,G) RP-bit Prune towards RP LATER?

Guessing at the topology you are describing:

           |
          2RP
           |
      -+---+---+-
       |       |
     2src     2rcvr
       |       |

2rcvr would have sent a (*,G) join toward the RP via 2RP
2RP would have received an (S,G) join from its RPF neighbor toward the RP.
This creates a case where the (S,G) route has the incoming interface (IIF)
   in the olist.  (It may not actually be there depending on the implentation)
On cisco routers, this would set the X-flag on 2RP which means that router
   will send (S,G) joins toward the source (via 2src) for 3 minutes.
If the last-hop router is configured to stay on the shared-tree, 2rcvr will
   continue to send only (*,G) joins toward the RP, which resets the X-flag
   on 2RP.
If the last-hop router is configured to join the shortest-path, in most
   implementations it has to wait until it receives data on the (S,G)
   IIF.  As soon as it does, that last hop will trigger the (S,G,R) prune,
   but more importantly, will not accept and forward traffic received on
   the RPF interface to the RP.

In any case, to get duplicate BW on a link, you'd have to have two 
routers forwarding to that LAN.  This will always result in the assert
process which will select only one forwarder.

All this means, you will not see duplicate BW usage.

> _______________________________________________
> Pim-external mailing list
> [email-masked]
> http://mailist.procket.com/mailman/listinfo/pim-external