c-bsr

On a PIM-SM network, the BSR mechanism is used to dynamically elect a Rendezvous Point (RP). The BSR collects Candidate-Rendezvous Point (C-RP) information to form an RP-set, and then advertises the RP-set to the entire network. Based on the RP-set, all Routers in the network can calculate out the RP to which a specific multicast group corresponds. The BSR and other devices in the PIM-SM domain need to exchange a great deal of information and thus lots of bandwidth should be reserved between the C-BSR and other devices in the PIM-SM domain.

One Router can function as both a C-RP and a C-BSR. One Router can be configured with multiple instances. Each instance can be configured with a maximum of one C-BSR. If the c-bsr command is run repeatedly in an instance, only the latest configuration takes effect.

One or more C-BSRs need to be configured in a PIM-SM domain. The C-BSRs automatically elect a BSR that is responsible for collecting and advertising RP information. C-BSRs must be Routers on the backbone network. When the Router is configured as a C-BSR, an interface enabled with PIM-SM needs to be specified.

The process of electing a BSR from C-BSRs is as follows:

• At first, each C-BSR considers itself as the BSR of the PIM-SM domain, and uses the interface IP address specified by using the c-bsr command as the BSR address to send Bootstrap messages.
• When a C-BSR receives a Bootstrap message from another Router, it compares the BSR information in the received Bootstrap message with the acknowledged BSR information in terms of priorities and IP addresses. In this process, the BSR with the higher priority is preferred. In the case of the same priority, the BSR with the higher IP address is preferred. If the BSR carried in a received the Bootstrap message is better than the current BSR, the C-BSR replaces its BSR address with the address of the BSR carried in the received Bootstrap message. Otherwise, the C-BSR keeps its BSR address and continues to consider itself as the BSR.