The PIM Assert serves a very specific use case in multicast where a
shared segment exists and the best path between the shared tree and the
source tree could be through different routers.
Let's dig into the PIM Assert packet from the designated router,
a router that asserts a better path.
The PIM Assert is message type 5.
The PIM options for the assert include the requested multicast group,
which is 239.3.2.1, and the source 10.99.1.100.
Notice that the designated router flags itself as using the
RP shared tree before it lists its metric.
The metric is based on the unicast best path metric to the source.
This assert message is sent to all PIM routers on the segment
essentially saying for this (S,G) entry,
this is my best path.
Now let's look at the winning PIM Assert that has the better
metric. As it is also an assert message, the format is
identical, with just a few key differences.
MCAST3 reports it is not connected to a shared tree, which is
accurate, as MCAST4 is the DR for the segment in part of the
shared tree flow. Also, look at the metric.
It's 3328, which is far less than the MCAST4 metric of 130816.
This is also sent to all PIM routers in the segment, and upon
receiving this PIM Assert, MCAST4 will stop being the designated router for this particular multicast (S,G) pair.