Overview

Bootstrap Router (BSR) is a multi-vendor PIMv2 protocol that multicast routers use to learn Rendezvous Point information for a specific multicast group. These multicast routers then use this information to select an RP.

The BSR Domain contains:

  • Bootstrap Router(s) – Advertises the group-to-RP mapping cache to all multicast routers so they can select their RP.
  • Candidate-RPs – These are PIM configured routers that send PIM messages to advertise RP eligibility to the BSR.

RP Election

The Network Administrator configures desired PIM routers to be Candidate-RPs for a specific multicast group.

The C-RPs send the BSR their information and the BSR stores their information in its group-to-RP mapping cache. This group-to-RP mapping cache is flooded, hop-by-hop, to every router in the network. Every multicast router will learn the address and priority of all C-RP routers.

The multicast routers make the RP selection for the multicast group based on the following criteria:

  1. The longest prefix of the multicast group address is preferred. This means that a /128 route is preferred over a /127 route and a /127 is preferred over a /126 and so on.
  2. If multiple C-RPs advertise the same prefix, then the priority will become the tie-breaker. The C-RP with the highest priority will win (0 is the highest priority value).
  3. If steps 1 and 2 are equal, then the tie-breaker will be the highest value that results from a hash function. This hash function is described in detail in RFC 2362.
  4. If all of the above are a tie, then the RP with the highest IP addressed interface to the PIM router will win.

If the RP goes down, the BSR’s group-to-RP mapping table will be updated to reflect the lost RP and be distributed across the entire network. Each multicast router will then select its new RP.

BSR Election

There can also be redundancy with BSRs.

Candidate-BSRs send PIM-BSR messages to notify every multicast router, via link-local multicast, the priority of each C-BSR for the specific multicast group.

The Candidate-BSR with the highest priority value (255 is the highest) will become the BSR. Tie-breakers are won by the C-BSR with the highest IP address.

All multicast routers in the network are notified as to which router has been elected the BSR. The BSR receives unicast Announcement messages from each C-RP.

BSR Flooding is used by non-PIM routers to flood BSR messages without processing them with.

Pro Tips:

  • It’s important to carefully plan which routers are RP/BSR candidates and what their priorities are.
  • Each router configured as a C-RP should also be configured as a C-BSR.

RFC 5059: Bootstrap Router (BSR) Mechanism for Protocol Independent Multicast (PIM)

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