F3726, F3211, F3174, R5135, R3816-HP Firewalls and UTM Devices Appendix Protocol Reference-6PW100

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, eac

h 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. 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

and RPs. The BSR then encapsulates the RP-set in the BSMs that it periodically originates and floods the

bootstrap messages to the entire PIM-SM domain.

Figure 55 BSR and C-RPs

Based on the information in the RP-sets, all routers in the network can calculate the location of the

corresponding RPs based on the following rules:

1. The C-RP with the highest priority wins.

2. If all the C-RPs have the same priority, their hash values are calculated through the hashing

algorithm. The C-RP with the largest hash value wins.

3. If all the C-RPs have the same priority and hash value, the C-RP that has the highest IP address wins.

The hashing algorithm used for RP calculation is "Value (G, M, C

) = (1103515245 * ( (1103515245 * (G

& M) + 12345) XOR C

) + 12345) mod 2