H3C Technologies H3C S10500 Series Switches - Manual (Page 146)

131
Figure 42 DR election
Join message
RP DR
DR
Hello message
Register message
Source
Receiver
Receiver
As shown in Figure 42 , the DR election process is as follows:
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 a hello message triggers a new DR election process among the
other routers.
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 you can statically specify the position of the RP 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 must be forwarded through the RP. To lessen the RP burden and
optimize the topological structure of the RPT, you can configure multiple candidate-RPs (C-RPs) in a
PIM-SM domain, among which an RP is dynamically elected through the bootstrap mechanism. Each
elected RP serves a different multicast group range. For this purpose, you must configure a bootstrap
router (BSR). The BSR serves as the administrative core of the PIM-SM domain. A PIM-SM domain can
have only one BSR, but can have multiple candidate-BSRs (C-BSRs). If the BSR fails, a new BSR is
automatically elected from the C-BSRs to avoid service interruption.
NOTE:
•
An RP can serve multiple multicast groups or all multicast groups. Only one RP can serve a given
multicast group at a time.
• A device can serve as a C-RP and a C-BSR at the same time.
As shown in Figure 43 , each C-RP periodically unicasts its advertisement messages (C-RP-Adv messages)
to the BSR. A C-RP-Adv message contains the address of the advertising C-RP and the multicast group
range that it serves. The BSR collects these advertisement messages and chooses the appropriate C-RP
information for each multicast group to form an RP-set, which is a database of mappings between