Enterasys Networks Security Router X-PeditionTM User Manual
Page 185
Describing the XSR’s PIM-SM v2 Features
XSR User’s Guide 7-9
interconnects with a router which is already on the shortest path tree from S to the same multicast
group, the Join message can end on that router to get a short-cut path.
After the path is established, both the native packet along the SPT tree and Register encapsulated
packet will be received by RP. When RP gets two copies of the same packet - one native and one
encapsulated packet - it drops the encapsulated packet and sends a RegisterStop message back to
the DR of S to let it know that it can now stop encapsulating packets in Register packets. After the
DR of S receives the RegisterStop message, it starts a RegisterStop timer and stops encapsulating
the packets until the RegisterStop timer expires.
At the end of phase two, packets flows natively from the sender to RP along the SPT tree, and then
are passed by RP to all the receivers along the shared distribution tree., as illustrated in
Figure 7-3 Phase 2 Topology: Shortest Path Tree Between Sender and RP
Phase 3: Building Shortest Path Tree Between Sender & Receiver
At the start of phase 3, the path from the sender to RP and on to the receiver is still not optimal for
sender or receiver since some receivers may want to optimize the path between themselves and
specific senders. In PIM-SM, the DR for the receiver has the flexibility to initiate a process to join
the shortest path tree from the sender to the multicast group. To do so, the DR for the receiver
desirous of optimizing the multicast path from sender S will send a source-specific Join message
towards S. The path passed by the Join message will be pinned especially for traffic from S to the
specific multicast group. After the SPT tree is set up between S and the DR of the receiver, traffic
from S to that multicast group will flow both on the SPT tree and the shared RP tree. When the
receiver can receive the same packet from both trees, the DR or upstream router for the receiver
begins to drop the packet from the shared RP tree and sends a Prune message to tell the RP tree
not to forward the traffic from the sender S for the specific multicast group.
After phase three, traffic from the sender will flow natively along the shortest path tree to the
receiver, as shown in
RP
Native Packet
Receiver
Receiver
Sender
Native Packet
Native
Packet
Register Stop
Native Packet
Register
Packet
RP
Native Packet
Native Packet
Receiver
Receiver
Sender
Native Packet
Native
Packet