Phase 1: Building A Shared Tree; Phase 2: Building Shortest Path Tree Between Sender & Rp - Enterasys Security Router X-PeditionTM User Manual

Describing the XSR's PIM-SM v2 Features

Phase 1: Building a Shared Tree

During phase one, PIM-SM builds a shared tree rooted at a special router called Rendezvous Point
(RP), as shown in
Designed Routers (DR) of the receivers of the group send their join requests. All PIM-SM enabled
routers within the PIM domain share uniform mapping between the multicast group and RP.
When a host wants to join a multicast group by using a group management protocol such as
IGMP, the elected DR on its subnet will send a PIM Join message towards the RP of that multicast
group by consulting the MRIB. The Join message will be processed hop by hop, and the path state
will be pinned in the intermediate routers. The Join message will end on RP or a router already
within the group. The paths the Join messages passed converge on RP forming a loop-free
multicast distribution tree, which is shared by all multicast group receivers.
Periodically, the Join message is re-sent to upstream routers for RP to keep the router on the
multicast tree. If a timeout occurs and there is still no Join message received by the upstream
router, the Join state for that multicast group will be removed from the upstream router. If all
receivers on the subnet leave the multicast group, the DR on that subnet will send a Prune message
towards RP to remove itself from the multicast tree for that multicast group.
Senders are not required to be on the shared multicast tree. The DR on the subnet of the senders
will encapsulate packets sent by the sender to the multicast group into a Register packet. Then the
Register packet will be sent by the DR to RP for that multicast group as a unicast packet. When RP
receives the Register packet, RP will decapsulate and send it down the shared multicast
distribution tree.
At the end of phase one, packets sent from senders will be unicasted to RP in encapsulated
Register packets and the native packets multicast over the shared distribution tree down to the
receivers.
Phase 2: Building Shortest Path Tree Between Sender & RP
Unicasting Register packets from multicast senders to RP is not efficient because:
• Encapsulating/decapsulating packets eats up router processing power
• Unicast routes may take a longer path from the sender to the multicast tree
Usually, RP will initiate the process to build the shortest path tree between a sender and RP. When
RP gets a Register packet from the DR of sender S, RP will send a Join message towards the S for
that specific multicast group. The path passed by the Join message is pinned. If the path
7-8 Configuring PIM-SM and IGMP
Figure 7-2. Each multicast group is mapped to a specific RP to which all
Figure 7-2 PIM-SM Phase 1 Topology: Shared Tree