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DES-7200 Configuration Guide Chapter 6 PIM-SM
Configuration
6-2
along this tree, the RP will send the registration stop message to the DR of the
source, notifying the DR of stopping registration. Consequently, the source’s
multicast data packets are sent to the RP along its shortest path tree rather than
being registered and encapsulated. Then the RP forwards the data packets to
group members along the shared tree. When there is no need of multicast data
packets, the DR multicasts a prune message to the RP of the group G hop-by-hop
to prune the shared tree.
The PIM-SM also offers a mechanism of select the root point (RP). One or more
Candidate-BSRs are configured in a PIM-SM domain. The PIM-SM selects a BSR
by following a certain rule. There are also Candidate-RPs in a PIM-SM domain
that unicast the packets including their IP addresses and available multicast
groups to the BSR. The BSR will periodically generate a BSR message which
includes a system candidate RP and the corresponding multicast group address.
The BSR messages are sent hop-by-hop within the whole domain. The device
receives and saves these BSR messages. If the DR receives a report on the
member relationship of a multicast group from its directly connected host but has
no route entries of the multicast group, the DR will use one Hash algorithm to map
the multicast group address to a candidate RP that can serve this group. Then,
the DR multicasts the Join/Prune message to the RP hop-by-hop. If the DR
receives multicast data packets from its directly connected host but has no route
entries of the multicast group, the DR will use one Hash algorithm to map the
multicast group address to a candidate RP that can serve this group. Then the DR
encapsulates multicast data packets into the registration message and unicasts it
to the RP.
The main difference between the PIM-SM and the broadcast/prune model-based
PIM-DM is that the PIM-SM is based on the explicit join model. In other words, the
receiver sends the join message to the RP, while the router only forwards the
packets of that multicast group on the outbound interface that has joined a
multicast group. The PIM-SM uses the shared tree to forward multicast packets.
Each group has a Rendezvous Point (RP). The multicast source sends the data
to the RP along the shortest path, and then the RP sends the data to the receivers
along the shortest path. This is similar to the CBT, but the PIM-SM does not use
the concept of core. One of the major advantages of the PIM-SM is that it not only
receives multicast messages through the shared tree but also provides a shared
tree-to-SPT conversion mechanism. Such conversion reduces network delay and
possible congestion on the RP, but it consumes enormous router resources. So it
is suitable for the case where there are only a few multicast data sources and
network groups.
The PIM-SM uses the shared tree and SPT to distribute multicast frames. At this
time, it is assumed that other devices don’t want to receive these multicasts
unless otherwise stated definitely. When a host joins a group, the equipment
connected to the host must notify the root (or the RP) by using the PIM join
message. This join message is transferred one after another through the routers
to create a shared tree structure. Therefore, the RP records the transfer path and
also the registration message from the first hop router (DR) of the multicast
source, and improves the shared tree upon these two messages. The branch/leaf
messages are updated by periodically querying messages. With the shared tree,
the multicast source first sends multicast packets to the RP, guaranteeing that all
the receivers can receive them.
The PIMv2 BSR is a method of distributing the group-to-RP message to all
devices without the need of setting the RP for them. The BSR uses the
hop-by-hop broadcast BSR message to distribute the mapping message. At first,
the BSR is selected among routers in the same process as selecting a root bridge
based on priority level among layer 2 bridges. Each BSR checks the BSR
messages and only forwards those having a priority higher than or equal to its
own (higher IP address). The selected BSR sends its BSR message to the
all-PIM-routers multicast group (224.0.0.13), where TTL is 1. After the adjacent