Multicast routing with PIM is among networking's most
difficult and technically nuanced topics.
I'm going to provide a high-level explanation of how it
works here for contextual awareness.
I am intentionally leaving out some technical details.
My goal is to communicate the general idea.
First,
the concept of a rendezvous point or RP is to link up
multicast receivers with sources.
Because receivers often don't know the sources ahead of time,
they just tell the RP that they want multicast.
Next,
I'll be using blue arrows to show PIM signaling and orange
arrows to show multicast traffic flow.
Let's get into the signal flow.
As we learned in previous clips,
the client signals interest in a multicast group using an
IGMP membership report or join message.
PIM translates this IGMP join into a PIM join and sends it toward the RP.
This introduces the concept of reverse path forwarding, or RPF.
In a sentence,
RPF is the path from a unicast routing perspective back
towards the source or rendezvous point.
At some point, the source will start sending traffic.
There is no PIM or IGMP signaling between sources and their first hop routers.
The first hop router sends a PIM register message to the RP,
which says I have an active source.
If the RP has interested receivers, which it does in this case,
it issues a PIM join towards the server using the RPF mechanism.
Once the full path is built,
traffic can flow all the way to the interested receiver.
In this course,
we are laser focused on what happens between the client and the last hop router, so don't worry if the PIM specifics are still fuzzy.