User's Manual

Table Of Contents
522
Users Manual of CS-6306R
Figure 5-1 Join-in mechanism of PIM-SM
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