Subject: [pim] RPT - SPT cross roads.

From: "Bakar" <[email-masked]>
Date: 11 Jul 2002 06:44:38 -0000

Hi,

   I couldnt come to conclusion on what should be the behaviour 
of
PIM in a particular scenario.

        +-------+      +-------+
-if1---|   A   |--if2-|   B   |---if4--
        |_______|      |_______|
             |              |
            if3            if5
             |              |

B rx (*,G) join on if5, sends (*,G) join to A on if2.
A sends (*,G) join on if1 towards RP.

A rx (S,G) join on if3, and forwards (S,G) join on if2 to B, and 
B
forwards (S,G) join on if4 towards S.

when data comes via RPT for S, the data at A is forwarded only 
on
if2 and B forwards on if5 only though
A and B has (S,G) join states on if3 and if2 they are not
forwarded for the simple reason that it is RPT data.
This fine.

Lets say Data comes via SPT also for S. when B first rx data on
if4, it sets the SPTbit and starts to forward data on if2 and on
if5. B also sends (S,G,rpt) prune on
if2 to A.

This will lead to a assert situation, even though we are not on 
a
multi access LAN.

I am not very clear on how the scenario proceeds from here.

Is the forwarding of RPT data on an (S,G) IIF acceptable ? 
Should
A in the first place should'nt have forwarded the RPT data on 
if2
?

regards,
Bakar

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---

From: "O.L.N.Rao" <[email-masked]>
Date: Thu, 11 Jul 2002 13:52:21 +0530

Hi Bakar,

    Protocol Overview -> Multi-access Transit LANS : States,
        "Assertion comes into picture only when
        there is more than one upstream router".

    Assertion do not occur on if2 in this case as there are no two
    upstream routers for B. 

Thanks & Regards, O.L.N.Rao

---

From: John Zwiebel <[email-masked]>
Date: Sat, 13 Jul 2002 19:46:22 -0700

>   Assertion do not occur on if2 in this case as there are no two

In this topology, you do have two upstream routers.  One is upstream
On the SPT and the other is upstream on the RPT.  The RPT always
Loses to the SPT, but there is still an assert that takes place.

Remember the T-bit.  It belongs to the last hop router.  Any
Router upstream that decides to not forward traffic down the RPT
Without having received a (S,G,R) prune is taking the T-bit decision
Away from the last hop -- potentially cutting off that receiver if
Data never flows down the shared-tree, which is one of the possibilities
Here if we follow the suggestion.

And as far as the receiver down the if3 branches, he doesn't care
Whether it got to A on the RPT or SPT he just wants the packet and
Wants it now.

When the assert happens A loses because its on the RPT.  At that
Point A creates (S,G) state and prunes the RPT.

Traffic for all receivers will now be delivered down the SPT to
Routers A and B.

What about the receiver on if5 you ask, its on the (*,G) tree.
So, B has (S,G) state with an inherited interface (if5).  It
Stays that way until the  receiver on if5 joins the SPT and
Sends (S,G,R) prunes up to RTR b.

WRT the question of forwarding data from the RPT on an S,G route,
Is the T-bit set?
z