are rooted at the rendezvous point. Source trees achieve the optimum path between each receiver and the
source at the expense of additional routing information: an (S,G) routing entry per source in the multicast
routing table. The shared tree provides a single distribution tree for all of the active sources. This means that
traffic from different sources traverse the same distribution tree to reach the interested receivers, therefore
reducing the amount of routing state in the network. This shared tree needs to be rooted somewhere, and the
location of this root is the rendezvous point. PIM BIDIR uses shared trees as their main forwarding mechanism.
The algorithm to elect the designated forwarder is straightforward, all the PIM neighbors in a subnet advertise
their unicast route to the rendezvous point and the router with the best route is elected. This effectively builds
a shortest path between every subnet and the rendezvous point without consuming any multicast routing state
(no (S,G) entries are generated). The designated forwarder election mechanism expects all of the PIM neighbors
to be BIDIR enabled. In the case where one of more of the neighbors is not a BIDIR capable router, the election
fails and BIDIR is disabled in that subnet.
PIM Shared Tree and Source Tree (Shortest Path Tree)
In PIM-SM, the rendezvous point (RP) is used to bridge sources sending data to a particular group with
receivers sending joins for that group. In the initial setup of state, interested receivers receive data from senders
to the group across a single data distribution tree rooted at the RP. This type of distribution tree is called a
shared tree or rendezvous point tree (RPT) as illustrated in
Figure 3: Shared Tree and Source Tree (Shortest
Path Tree), on page 13
. Data from senders is delivered to the RP for distribution to group members joined
to the shared tree.
Figure 3: Shared Tree and Source Tree (Shortest Path Tree)
Unless the
spt-threshold infinity
command is configured, this initial state gives way as soon as traffic is
received on the leaf routers (designated router closest to the host receivers). When the leaf router receives
Cisco IOS XR Multicast Configuration Guide for the Cisco CRS Router, Release 5.2.x
13
Implementing Multicast Routing on Cisco IOS XR Software
PIM Shared Tree and Source Tree (Shortest Path Tree)