For this packet analysis, we'll be focusing on the interaction
between different packets more than any individual one.
Recall that the behavior in PIM sparse mode is for a Last Hop Router to
send in any‑source PIM join towards the rendezvous point in hopes of
finding out if any sources are sending to that group.
When the PIM join reaches the RP,
traffic flows down the shared tree to the Last Hop Router. And
once the Last Hop Router learns about the source,
it will initiate what's called the shortest path tree switchover.
This is where the LHR sends a source tree PIM join directly to the source,
which should create an optimal path back from source to receiver.
We'll tackle the packets in order of operation. You've seen them before
in other modules. I want to focus on what's important to understand the
process rather than list every field.
This PIM join is the initial any‑source join sent from the Last Hop Router.
It's important to pay attention to the requested source.
It's the IP of the RP, 1.1.1.1.
The receiver didn't ask for this source. The Last Hop
Router inserted it because the PIM joins need to be
forwarded towards the rendezvous point.
That source IP allows resolution of the incoming reverse path forward
interface to be determined by the unicast best path.
Once the Last Hop Router learns the source,
it sends a new PIM join, this time towards the source of the traffic itself.
This new PIM join is tracked separately in the mroute table and may
have a completely different incoming RPF interface.
Now we see the PIM prune being sent upstream to cut off the shared flow.
This upstream router will remove the interface from its outgoing interface list,
and if no other interfaces are in its own OIL,
it will forward its own PIM prune up towards the rendezvous point until traffic is no longer being delivered along the shared tree.