3Com Switch 7754, Configuration Manual

Page: 467 / 940

PIM Overview 467
One or more candidate BSRs must be configured in a PIM domain. Through the
auto-election, the candidate BSRs elect a BSR which is responsible for collecting
and advertising RP information. The auto-election among candidate BSRs is
described in the following section:
■
Specify a PIM-SM-enabled interface when configuring a router as a candidate
BSR.
■
Each candidate BSR considers itself as the BSR of the PIM-SM and uses the IP
address of the specified interface as the BSR address to send Bootstrap
messages.
■
When the candidate BSR receives Bootstrap messages from other routers, it will
compare the BSR address in the received Bootstrap message with its own BSR
address in priority and IP address. When the priority is the same, the candidate
BSR with a higher IP address is considered to be better. If the former is better,
the candidate BSR will replace its own BSR address with the new BSR address
and does not consider itself as BSR any more. Otherwise, the candidate BSR will
keep its own BSR address and continue to consider itself as BSR.
The positions of RPs and BSRs in the network are as shown in Figure 115:
Figure 115 Diagram for the communication between RPs and BSRs
Only one BSR can be elected in a network or management domain, while multiple
candidate BSRs (C-BSR) can be configured. In this case, once the BSR fails, other
C-BSRs can elect a new BSR through auto-election. Thus, the service is prevented
from being interrupted.
In the same way, multiple C-RPs can be configured in a PIM-SM domain, the RP
corresponding to each multicast group is worked out through the BSR mechanism.
RPT building
Assume the receiver hosts are User B, User D, and User E. When a receiver host
joins in a multicast group G, it will inform the leaf router directly connected to the
host through IGMP packets. Thus the leaf router masters the receiver information
of the multicast group G, and then the leaf router will send Join messages to the
upper-layer nodes in the direction of RP, as shown in Figure 116:
Advertisement message
BSR message
PIM-SM
BSR
C-RP
C-RP
C-RP
C-BSR