A static RP configuration works for a small, stable PIM network domain; however, it is not practical for a large and/or complex one. In such a network, if the RP fails or you have to change the assignment of the RP, you are required to reconfigure the static configurations on all PIM routers. Also, if you have several multicast groups mapped to several RPs, there are many repetitive configurations you are required to perform, which can be time consuming and laborious. Thus when it comes configuring RP in large and/or complex networking environments, configuring it dynamically is the best and most scalable method to use. Bootstrap router (BSR) configuration is one method of configuring the RP dynamically.

The BSR mechanism in a PIM domain uses the concept of a RP as a way for receivers to discover the sources that send to a particular multicast group. The BSR mechanism gives a way for a multicast router to learn the set of group-to-RP mappings required in order to function. The BSR’s function is to broadcast the RP set to all routers in the domain.

Some of the PIM routers within a PIM domain are configured as Candidate-RPs (C-RPs). A subset of the C-RPs is eventually used as the actual RPs for the domain. An RP configured with a lower value in the priority field has a higher priority.

Some of the PIM routers in the domain are configured to be Candidate-BSRs (C-BSR). One C-BSR is selected to be the BSR for the domain, and all PIM routers in the domain learn the result of this election through Bootstrap messages (BSM). The C-BSR with highest value in the priority field is elected to be the BSR. The C-RPs then report their candidacies to the elected BSR, which chooses a subset of the C-RPs, and distributes corresponding group-to-RP mappings to all the routers in the domain using Bootstrap messages.

This section provides 2 examples to illustrate the BSR configuration for configuring RP dynamically.

For this example, refer to Figure 1 for the topology.

To dynamically configure the RP, Router_C on eth1 and Router_D on eth1 are configured as a Candidate RP using the ip pim rp-candidate command. Router_D on eth1 is also configured as the Candidate BSR. Since no other router has been configured as the candidate BSR, Router_D becomes the BSR router and is responsible for sending group-to-RP-mapping information to all other routers in this PIM domain.

The highest priority router (configured with lowest priority value) is chosen as the RP. If two or more routers have the same priority, a hash function in the BSR mechanism is used to choose the RP to ensure that all routers in the PIM-domain have the same RP for the same group.

To change the default priority of any candidate RP, use the ip pim rp-candidate IFNAME PRIORITY command. At Router_D, the show ip pim rp mapping command shows that Router_C is chosen as the RP for a specified group.

 

 

The following output displays the complete configuration at Router_C and Router_D:

This section provides the steps to verify the RP configuration.

The show ip pim rp mapping command displays the group-to-RP mapping details and displays information about RP candidates. There are two RP candidates for the group range, 224.0.0.0/4. RP Candidate 10.10.1.5 has a default priority of 192, whereas, RP Candidate 172.16.1.2 has been configured to have a priority of 2. Since RP candidate 172.16.1.2 has a higher priority, it is selected as RP for the multicast group, 224.0.0.0/4.

To display information about the RP router for a particular group, use the following command. This output displays that 172.16.1.2 has been chosen as the RP for the multicast group 224.0.1.3.

After RP information reaches all PIM routers in the domain, various state machines maintain all routing states, as a result of Join/Prune from group membership. To display information on interface details and the multicast routing table, see Configuring Rendezvous Point Statically.

To dynamically configure the RP, Router_2 on eth1 is configured as a Candidate RP using the ip pim rp-candidate command. Since no other router is configured as C-RP, Router_2 becomes the RP. Router_1 on eth1 and Router_2 on eth1 are configured as the Candidate BSRs. Since Router_1 has a higher priority value than Router_2, Router_1 becomes the BSR router and is responsible for sending group-to-RP-mapping information to all other routers in this PIM domain.

For this example, refer to Figure 3-4 for the topology.

When the ip pim unicast-bsm command is configured on an interface that is a DR for a network, then that interface unicasts the stored copy of BSM to the new or rebooting router.