From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Mon Jun 25 2007 - 17:07:14 ART
Hi John,
Have a look at section 3 of RFC4601. I think it provides a very good
explanation of this.
Paul.
John Gibson wrote:
> Paul, you are right. I only need to use the command
> at the leaf. Not the branches.
>
> The branch routers clean up (S,G) by themselves.
> I don't understand prune. I am looking it up.
>
> John
>
> --- Paul Cosgrove <paul.cosgrove@heanet.ie> wrote:
>
> The command only needs to be applied to leaf routers but it may well
> affect the multicast routing tables on other routers.
>
> When a host requests traffic for a multicast group, a PIM-SM router on
> its local subnet will initially send a PIM Join for that group towards
> the RP. The router does not yet know which hosts are sources of that
> group, but it will always know the RP (because of static definition,
> AutoRP or BSR). The RP is introducing receivers last hop routers and
> sources, passing on requests for traffic and then sending the
> resulting mutlicast stream back towards the receivers along the (*,G)
> tree.
>
> Although an RP is necessary in PIM-SM, it has the drawback of being a
> potential bottleneck, is often a single point of failure and it may
> introduce extra hops between the sender and receivers. Once a last
> hop router begins receiving multicast data for the group, it makes a
> note of the source address of the packets. It then has the option of
> joining a direct (S,G) tree back to the sender and cutting out the
> middle man.
>
> ip pim spt-threshold controls if (and when) the last hop router
> changes to sending (S,G) requests direct to the source. If it is
> configured to changeover, then once it begins receiving the multicasts
> on the direct path (S,G), it will then send a (*,G) prune request
> towards the RP.
>
> If routers between the last hop router and the RP have had no other
> (*,G) requests for the traffic, then the prune will be propagated by
> each until it reaches the RP. The multicast routing table on each of
> these will change as a result.
>
> Paul.
>
>> johngibson1541@yahoo.com wrote:
>>
>>> I know this command is used to reduce those (S,G)
>>>
>> entries.
>>
>>> But when I tested this, not only the "leaf router"
>>> (immediately adjacent to the receiver) is
>>>
>> effected,
>>
>>> but also my branch router down from my RP.
>>>
>>> It makes sense to me if univercd said this command
>>>
>> be
>>
>>> applied to all multicast routers and all branch
>>> routers down the RP and leaf router are effected.
>>>
>>> I will assume this is a minor error in univercd.
>>>
>>>
>>>
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