IP PIM dense mode, Assert message on serial int.

[sarahr202]

Hi everybody.

Please consider the following example.

All routers are running IPpim dense mode. Sever is generating multicast traffic 224.7.7.7 continuously ( I use ping 224.7.7.7  repeat 100 millions)

No state refresh message is enabled.

Please focus on R3 and R2.  R3 s0/1 is pruned . R2 's s0/0 is pruned too.

401 418.518000 202.202.202.3 224.0.0.13 PIMv2 50 Assert

537 600.508000 202.202.202.3 224.0.0.13 PIMv2 50 Assert

Above  at 418 sec, R3 sends alert to R2.

Approximately after 3 minutes, R2 sends another " Assert"

This goes on.

My questions:

1) I thought PIM uses " Assert" message on LAN int. But here I find R3 sending " assert" message on serial link.

2) Every 3 minutes ,R3 should send prune message to R2(  R3 is receiving multicast traffic continuously) so R2 can reset its prune timer.

But I don't see any " Prune/Joint " message from R3. All I see is " assert" message from R3 every three minutes.

Thanks and have a great day.

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Below is the topology:

[sarahr202]

1) I thought PIM uses " Assert" message on LAN int. But here I find R3 sending " assert" message on serial link.

Just found out my book was wrong. According to my book, PIM router sends " Assert Message" on LAN interface.

In fact Router sends also send " Assert message" on serial link.

2) Every 3 minutes ,R3 should send prune message to R2(  R3 is  receiving multicast traffic continuously) so R2 can reset its prune  timer.

But I don't see any " Prune/Joint " message from R3. All I see is " assert" message from R3 every three minutes.

PIM routers send Prune message when there is no longer outgoing int for a a Specific ( S,G) entry.

In our example, both routers R2 and R3 will exchange " Assert message" . At end of these exchanged, both routers will Prune their interfaces to avoid waste of bandwidth.  This Punning of interfaces between R2 and R3 results because of " Assert Message" exchanges. 

One difference I noticed between " Assert message " exchange between two PIM routers on serial link and two Pim routers on LAN link is that on serial link the exchange of " Assert message"  results in pruning of interfaces on serial link. On the other hand, the exchange of " assert message" on LAN interface results in determining who will be responsible for forwarding multicast  Packets on the LAN for a particular group.

Thanks have a great day.