[Xorp-users] IGMP and PIM-SSM problem

[Pavlin Radoslavov]

> > The sender 192.168.14.14 is directly connected to both PC-A and
> > PC-B.
> If I change the IP address of source in IGMP report i.e. if 192.168.14.14 is
> changed to 11.22.33.44 then it is not directly connected to both PCs. Is there
> any other modifications required to prove that source is not directly
> connected?

The IGMP reports are used by the receivers to indicate they want to
join a multicast group (or source-group in case of SSM) so they
won't affect the senders.

If you change the IP address of the sender so it doesn't belong to
the XORP subnets, and the sender is still connected to the same LAN,
then yes, from XORP perspective it won't be considered directly
connected.
However, XORP still won't send PIM-SM Join message toward the
sender, because the Join message will be sent to a directly
connected PIM-SM router that is the next-hop (from MRIB perspective)
toward the sender.

Just curious, why do you want to force XORP to send PIM-SM Join
message toward the directly connected sender, when from protocol
perspective it is not necessary?
Typically, the multicast sender starts transmitting even if there
are no receivers, and the directly connected PIM-SM routers will
forward the traffic only if there are downstream receivers.

> > Hence, the PIM-SM router connected to it will NOT
> > send PIM-SM Join message toward it. PIM-SM Join messages are only
> > sent to PIM-SM routers toward the sender (or the RP), and only if
> > the sender (or the RP) is not directly connected.
> >
> > Pavlin
> >
> Can you provide me the reference in RFC for this particular statement because
> I am not aware of this rule.

The above statement is a summary of the following PIM-SM rules:

* PIM-SM Join messages are sent only to PIM-SM neighbors, which by
  definition are directly connected.
* PIM-SM Join messages are sent on the RPF (Reverse Path Forwarding)
  toward the RP or the sender (depends on the type of the Join).
* The PIM-SM (S,G) Join message is not propagated further once it
  reaches a PIM-SM router that already has (S,G) Join state or the
  PIM-SM router that is directly connected to the sender.

All that information is in RFC 4601 (the PIM-SM spec).
Within the context of directly-connected sources, the following
statement from the spec is important:

   The function MRIB.next_hop( S ) returns an address of the next-hop
   PIM neighbor toward the host S, as indicated by the current MRIB.  If
   S is directly adjacent, then MRIB.next_hop( S ) returns NULL.


Hope that helps,
Pavlin