HP VPN Firewall Appliances Appendix Protocol Reference

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Figure 54 DR election
As shown in Figure 54, the DR election process is as follows:
1. Routers on the shared-media 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.
3. 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. An RP can serve multiple multicast groups or all multicast groups, but
a given multicast group can have only one RP to serve it at a time.
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 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 BSR.
A BSR serves as the administrative core of the PIM-SM domain. A PIM-SM domain can have only one BSR,
but can have multiple C-BSRs. If the BSR fails, a new BSR is automatically elected from the C-BSRs to
avoid service interruption. A device can serve as a C-RP and a C-BSR at the same time.
As shown in Figure 55, ea
ch C-RP peri
odically 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. It then chooses the appropriate C-RP information for
each multicast group to form an RP-set. The RP-set is a database of mappings between multicast groups