Re: can ssm and non-ssm colide ? (re-sent)

[Mark Handley <mjh@aciri.org>]

>http://www.ietf.org/internet-drafts/draft-ietf-pim-sm-v2-new-00.txt
>
>  At the end of phase 2, traffic will be flowing natively from S along a
>  source-specific tree to the RP, and from there along the shared tree to
>  the receivers.  Where the two trees intersect, traffic may transfer from
>  the source-specific tree to the RP tree, and so avoid taking a long
>  detour via the RP.
>
>and 
>
>  All of these problems are caused by there being more than one upstream
>  router with join state for the group or source-group pair.  PIM does not
>  prevent such duplicate joins from occurring - instead when duplicate
>  data packets appear on the LAN from different routers, these routers
>  notice this, and then elect a single forwarder.  This election is
>  performed using PIM Assert messages, which resolve the problem in favor
>  of the upstream router which has (S,G) state, or if neither or both
>  router has (S,G) state, then in favor of the router with the best metric
>  to the RP for RP trees, or the best metric to the source to source-
>  specific trees.
>
>
>I BELIEVE that if there is both (S,G) and (*,G) forwarding state,
>that the (S,G) list is searched first and then the (*,G) is used only if
>there is no (S,G) match, but I cannot (as I wait for the turkey to cook)
>find any documentation that confirms this.

It's in the "Data Packet Forwarding Rules" in Section 4.2.

A couple of provisos modify what you said above:

 - if there's (S,G) state, then the packet will be forwarding using a
   combination of both (S,G) and (*,G) state if it arrived on the
   correct interface to have come from S.

 - if the packet arrived on the correct interface to have come from
   the RP (and not from S), then it will only be forwarded using
   purely (*,G) state if the SPTbit for (S,G) is false (indicating the
   the (S,G) forwarding state is not yet active).

Cheers,
	Mark