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Bootstrap router or BSR is standard-based protocol available with PIMv2. The protocol performs the same function as Cisco’s proprietary Auto-RP, i.e. disseminates RP information. Both protocols use the concept of candidate RP, i.e the router that is announcing itself as a potential RP. Unlike Auto-RP, BSR does not use any dense-mode groups to flood candidate RP and RP mapping information. Instead, the information is flooded using PIM messages, on hop-by-hop basis. The flooding procedure utilizes reverse path forwarding: when a router receives a BSR message, it applies RPF check, based on the source IP address in the packet. If the RPF check succeeds, the message is flooded out of all PIM-enabled interfaces until all routers in the domain learn the information.

Let’s briefly discuss BSR functions. BSRs listen and accumulate candidate RP announcements, performing the role similar to Auto-RP MA. However, unlike Auto-RP MA, the BSR does not elect the best RP for every group range it learns about. Instead of this, for every group range known, the BSR builds a set of candidate RPs, including all routers that advertised their willingness to service this range. This is called “group range to RP set mapping”. The resulting array of group range to RP set mappings is distributed by the BSR using PIM messages and the same flooding procedure described above. PIM BSR operates in two phases:

Phase 1: BSR discovery. Every router configured as BSR floods bootstrap messages and listens to other BSR candidates. The BSR that hears another BSR with a higher priority gives up its role of the BSR. Eventually there is only one BSR, and every router in the domain, including the candidate RPs learn about it.
Phase 2: RP discovery. Every RP unicasts its own address and the configured group ranges to the BSR. Eventually, BSR learns of every RP and continuously keeps flooding the new information through the domain.

To set up a candidate RP in Cisco IOS, use the command ip pim rp-candidate < PIM-Enabled-Interface > [group-list ] [interval ] [priority <0-255>]. If you omit all optional arguments, the router will start advertising itself as the RP for all groups. You may specify the list of groups using the group-list argument. All entries in this list are “positive”, if you compare them to Auto-RP functionality. You cannot use “negative” groups, singnaling dense-mode forwarding. RP priority value is used only when the routers select the best RP for a given group, and lower values are preferred. The default RP priority is zero (which is against standard, which specifies 192) and is the highest possible value. You may rarely want to change the priority value for a candidate RP, possibly only in cases when you want to gracefully take the RP out of service.
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