Install guide

12 Software Reference
Shortest path tree This phase further optimises routing by using shortest path trees (SPT). In
phase 3 the receiver joins the shortest path tree between the source and
receiver. This allows a multicast group member to receive multicast data by the
shortest path from the sender, instead of from the shared RP tree. When the
receiver’s DR receives multicast data from a particular sender, it sends a join
message towards the sender. When this message reaches the sender’s DR, the
DR starts forwarding the multicast data directly towards the receiver. As
several receivers all initiate shortest paths to the sender, these paths converge,
creating a shortest path tree.
When the multicast packets start arriving from the SPT at the receiver’s DR or
an upstream router common to the SPT and the RPT, it starts discarding the
packets from the RPT, and sends a prune message towards the RP. The prune
message travels up the RPT until it reaches the RP or a router that still needs to
forward multicast packets from this sender to other receivers. Every time a
router receives a prune message, it waits a short time (the J/P Override Interval
specified in Internet Draft draft-ietf-pim-sm-v2-new-05) before putting the
prune into effect, so that other routers on the LAN have the opportunity to
override the prune message.
Multi-Access LANs If the PIM-SM network includes multi-access LAN links for transit, as well as
point-to-point links, then a mechanism is needed to prevent multiple trees
forwarding the same data to the same group member. Two or more routers on a
LAN may have different information about how to reach the RP or the
multicast sender. They could each send a join message to two different routers
closer to the RP for an RPT or the sender for an SPT. This could potentially
cause two copies of all the multicast traffic towards the receiver.
When PIM routers notice duplicate data packets on the LAN, they elect a single
router to forward the data packets, by each sending PIM Assert messages. If
one of the upstream routers is on an SPT and the other is on an RPT, the router
on the SPT has the shortest path to the sender, and wins the Assert election. If
both routers are on RPTs the router with the shortest path to the RP (the lowest
sum of metrics to the RP) wins the Assert. If both routers are on an SPT, then
the router with the shortest path to the sender (the lowest sum of metrics to the
sender’s DR) wins the Assert.
The router that won the Assert election forwards these data packets, and acts as
the local designated router for any IGMP members on the LAN. The
downstream routers on the LAN also receive the Assert messages, and send all
their join messages to the Assert winner. The result of an Assert election times
out after the Assert Time specified in the Internet Draft draft-ietf-pim-sm-v2-
new-05. As long as the situation causing the duplication remains unchanged,
the Assert winner sends an Assert message at a the Assert time interval, before
the previous Assert messages time out. When the last downstream router
leaves the SPT, the Assert winner sends an Assert Cancel message saying that it
is about to stop forwarding data on the SPT. Any RPT downstream routers then
switch back to the RP tree.