Operation Manual – Multicast Protocol
H3C S3610&S5510 Series Ethernet Switches
Chapter 6 PIM Configuration
6-8
1) Routers on the multi-access network send hello messages to one another. The
hello messages contain the router priority for DR election. The router with the
highest DR priority will become the DR.
2) In the case of a tie in the router priority, or if any router in the network does not
support carrying the DR-election priority in hello messages, the router with the
highest IP address will win the DR election.
When the DR fails, a timeout in receiving hello message triggers a new DR election
process among the other routers.
III. RP discovery
The RP is the core of a PIM-SM domain. For a small-sized, simple network, one RP is
enough for forwarding information throughout the network, and the position of the RP
can be statically specified on each router in the PIM-SM domain. In most cases,
however, a PIM-SM network covers a wide area and a huge amount of multicast traffic
needs to be forwarded through the RP. To lessen the RP burden and optimize the
topological structure of the RPT, each multicast group should have its own RP.
Therefore, a bootstrap mechanism is needed for dynamic RP election. For this purpose,
a bootstrap router (BSR) should be configured.
As the administrative core of a PIM-SM domain, the BSR collects advertisement
messages (C-RP-Adv messages) from candidate-RPs (C-RPs) and chooses the
appropriate C-RP information for each multicast group to form an RP-set, which is a
database of mappings between multicast groups and RPs. The BSR then floods the
RP-set to the entire PIM-SM domain. Based on the information in these RP-sets, all
routers (including the DRs) in the network can calculate the location of the
corresponding RPs.
A PIM-SM domain (or an administratively scoped region) can have only one BSR, but
can have multiple candidate-BSRs (C-BSRs). Once the BSR fails, a new BSR is
automatically elected from the C-BSRs through the bootstrap mechanism to avoid
service interruption. Similarly, multiple C-RPs can be configured in a PIM-SM domain,
and the position of the RP corresponding to each multicast group is calculated through
the BSR mechanism.
shows the positions of C-RPs and the BSR in the network.