PIM-SM - RPT-> SPT transition

Unanswered Question

Hi all,

suppose to have one network with OSPF as IGP and PIM-SM for multicast.

Suppose that a router (R0) is in a Stub area and know the RP and the Mcast Source with the same default route for RPF check.

After that Source start to send traffic, do you thik that RPT-> SPT transition should happens... actually we do not have a shorter tree... because R0 known RP and Mcast Source with the same cost...?!

I'd like to understand if R0 send prune message to RP or not.

Thank you!



I have this problem too.
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mikedavi1 Thu, 02/22/2007 - 14:53

Hello Graziano,

By default, SPT transition will occur, even if the path is identical as soon as the spt-threshold is met or exceeded. spt-threshold is defined.

Router(config)#ip pim spt-threshold ?

<0-4294967> Traffic rate in kilobits per second

infinity Never switch to source-tree

The default threshold is 0 so the spt transition occurs as soon as the first multicast packet is received from the rp, by the receiver's router.

So there are two Sparse-Mode situations where spt-transition should not occur:

1) spt-threshold is set to infinity.

Traffic will continue to tunnel via unicast from the source router to the RP, then be multicast down the shared tree to the receiver.

2) Pim bidir is enabled.

Multicast traffic will travel bi-directionally up/down the shared tree. I don't believe the unicast tunneling continues, but the path through the RP is used.




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