We'll talk very briefly about a redundancy strategy
baked into IPv6 PIM called Anycast RP, next.
Anycast is a way to refer to a service that is advertised
from multiple places in the network, usually using the same service address.
In this case,
we configure multiple rendezvous points with the same IP address
and advertise that IP address into the network.
The result is that each PIM router will use its Unicast routing table to send
traffic to whichever RP is closest per the metric; this provides RP redundancy
and quick failover should a path or router fail.
Something like this exists in IPv4 as well,
but uses a protocol called Multicast Source Discovery Protocol,
or MSDP, to work.
MSDP is not supported with IPv6,
so this solution was created specifically for IPv6.
Both work the same, however.
The issue with using an Anycast RP is that for sources
and receivers to find each other,
each must receive source registrations so that any PIM
joins will be able to build a shared tree.
In this example, the Unicast RIB has sent PIM joins to MCAST1 on the left,
and the source registration has gone to MCAST2 on the right.
The situation will never resolve itself because every router has
done its job and is unaware that there are two running endpoints The
way to resolve this issue is to configure a unique loopback on each
member of an Anycast RP set.
This unique loopback is also injected into the routing protocol and is only
used to share source registrations among members of the set.
This is important because a first‑hop router will only register
with one RP, and all RPs need to know where the sources are in
order to pair them with receivers.
Each Anycast RP on receiving a source registration will advertise the source to
its configured peers via the same PIM register method that sources use. The
peer Anycast RP will respond with a register stop,
and if they have any local PIM joins waiting for that group, they'll trigger their own source tree joins.