It's time to inspect the PIM packets in detail.
We'll start with the bootstrap router packets.
These packets are only originated if the BSR is configured.
The bootstrap messages are forwarded on every PIM
segment in a manner like PIM hellos.
This ensures all routers learn about the BSR and later
about the chosen rendezvous point.
The Bootstrap message is Type 4 and is used to advertise
both the bootstrap router itself and the RP, once chosen.
As we inspect the PIM options,
that should be clear. The BSR priority is configurable with higher being better.
What follows after that is a list of the BSR routers based on the interfaces
configured to offer the service. 2.2.2.2 is a loopback on MCAST2, which we
configured as the BSR, so that appears here.
Next comes the multicast range that an elected RP will service. In this lab,
it's the entire multicast range, but in a production network, there could be
multiple elected RPs servicing different group ranges.
The RP count is 1, as we only have one RP in the lab.
The FRP count is also 1, and this refers to the fragment RP count. In a large
network with many BSR and RP candidates, it's possible that the PIM bootstrap
message might exceed the maximum size of an allowed packet. And this is a way to
track that there are multiple messages, which will carry a fragment tag. This
allows a complete picture of all candidate RPs and BSRs to span multiple
bootstrap messages, if needed.
The last part of the bootstrap message lists the RPs and the unicast IP,
which is based on the chosen interface to provide the RP service. In this
lab, MCAST1 is using its loopback as the RP. Moving on to the candidate
RP packet, we can see that the destination is unicasts not sent to the
all PIM routers group.
The PIM message type is 8.
The prefix count is 1, which indicates that the candidate
RP is offering to provide RP services for one prefixed
range at a default priority of 0.
If the RP were servicing different ranges of multicast groups,
the prefix count would be equal to the number of
different ranges being advertised.
The RP address is 1.1.1.1, and this RP is offering to
provide the service for the whole range.
This information matches what the bootstrap router advertised in the bootstrap message that we looked at before.