User's Manual

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Chapter 75 Configuring PIM-SM
75.1 PIM-SM Introduction
Protocol Independent Multicast Spare Mode (PIM-SM) is a multicast routing protocol in sparse mode. In the
PIM-SM domain, the switches that run PIM-SM periodically send the Hello information to achieve the following
purposes:
 Discover neighboring PIM-SM switches.
 Select the designated router (DR) in the multi-access network.
The DR sends the join/prune message to the directly-connected group members at the direction of multicast
distribution tree, or sends the data of the directly-connected multicast source to the multicast distribution tree.
PIM-SM forwards the multicast packet by creating the multicast distribution tree. The multicast distribution tree
can be classified into two groups: Shared Tree and Shortest Path Tree. Shared Tree takes the RP of group G
as the root, while Shortest Path Tree takes the multicast source as the root. PIM-SM creates and maintains
the multicast distribution tree through the displayed join/prune mode. As shown in Figure 5-1, when the DR
receives a Join message from the receiving side, it will multicast a (*, G)-join message at each hop towards
the RP of group G to join in the shared tree. When the source host sends the multicast message to the group,
the packet of the source host is packaged in the registration message and unicast to the RP by the DR; The
RP then sends the unpackaged packet of the source host to each group member along the shared tree; The
RP sends the (S,G)-join message to the first-hop switch towards the source’s direction to join in the shortest
path tree of the source; In this way, the packet of the source will be sent to the RP along the shortest path tree
without being packaged; When the first multicast data arrives, the RP sends the registration-stop message to
the DR of the source and the DR stops the registration-packaged process. Afterwards, the multicast data of
the source is not packaged any more, but it will be sent to the RP along the shortest path three of the source
and then sent to each group member by the RP along the shared tree. When the multicast data is not needed,
the DR multicasts the Prune message hop by hop towards the RP of group G to prune the shared tree.
PIM-SM also deals with the RP choice mechanism. One or multiple candidate BSRs are configured in the PIM-
SM domain. You can select a BSR among candidate BSRs according to certain regulations. Candidate RPs
are also configured in the PIM-SM domain. These candidate RPs unicast the packets containing RP’s address
and multicast groups to the BSR. The BSR regularly generates the Bootstrap message containing a series of
candidate RPs and corresponding group addresses. The Bootstrap message is sent hop by hop in the whole
domain. The switch receives and stores the Bootstrap message. After the DR receives a report about a group
member’s relationship from the directly-connected host, if the DR has no the routing item of the group, the DR
will map the group address to a candidate RP through the Hash algorithm. The DR then multicasts the
Join/prune message hop by hop towards the RP. Finally, the DR packages the multicast data in the registration
message and unicasts it to the RP.