CANOGA PERKINS 9175 Configuration Guide - Manual (Page 148)

CanogaOS Configuration Guide
24-6
Determining the RP
PIM-SM uses a BootStrap Router (BSR) to originate Bootstrap messages, and to
disseminate RP information. The messages are multicast to the group on each link. If the
BSR is not apparent, the routers flood the domain with advertisements. The router with
the highest priority (if priorities are same, the higher IP address applies) is selected to be
the RP. Routers receive and store Bootstrap messages originated by the BSR. When a
DR gets a membership indication from IGMP for (or a data packet from) a directly
connected host, for a group for which it has no entry, the DR maps the group address to
one of the candidate RPs that can service that group. The DR then sends a Join/Prune
message towards that RP.
In a small domain, the RP can also be configured statically.
Joining the Shared Tree
To join a multicast group, a host sends an IGMP message to its upstream router, after
which the router can accept multicast traffic for that group. The router sends a Join
message to its upstream PIM neighbor in the direction of the RP. When a router receives
a Join message from a downstream router, it checks to see if a state exists for the group
in its multicast routing table. If a state already exists, the Join message has reached the
shared tree, and the interface from which the message was received is entered in the
Outgoing Interface list. If no state exists, an entry is created, the interface is entered in
the Outgoing Interface list, and the Join message is again sent towards the RP.
Registering with the RP
A DR can begin receiving traffic from a source without having a Source or a Group state
for that source. In this case, the DR has no information on how to get multicast traffic to
the RP through a tree. When the source DR receives the initial multicast packet, it
encapsulates it in a Register message, and unicasts it to the RP for that group. The RP
deencapsulates each Register message, and forwards the extracted data packet to
downstream members on the RPT.
Once the path is established from the source to the RP, the DR begins sending traffic to
the RP as standard IP multicast packets, as well as encapsulated within Register
messages. The RP temporarily receives packets twice.
When the RP detects the normal multicast packets, it sends a Register-Stop message to
the source DR, meaning it should stop sending register packets.
Sending Register-Stop Messages
When the RP begins receiving traffic from the source, both as Register messages and as
unencapsulated IP packets, it sends a Register-Stop message to the DR. This notifies
the DR that the traffic is now being received as standard IP multicast packets on the SPT.
When the DR receives this message, it stops encapsulating traffic in Register messages.
Pruning the Interface
Routers attached to receivers send Prune messages to the RP to disassociate the
source from the RP. When an RP receives a Prune message, it no longer forwards traffic
from the source indicated in the Prune message. If all members of a multicast group are
pruned, the IGMP state of the DR is deleted, and the interface is removed from the