PIM Sparse Mode Bootstrap Router

PIM sparse mode routers need the address of the rendezvous point (RP) for each group for which they have (*,G) state. They obtain this address either through a bootstrap mechanism or through static configuration. PIM sparse mode routers commonly use one of two bootstrap mechanisms: bootstrap router (BSR) or auto-RP. Auto-RP is standards based, but is not used in IPv6 implementations, so BSR configuration has become more popular.

When implemented, BSR operates as follows:

1. One router in each PIM domain is elected the BSR.
2. All the routers in the domain that are configured to be RP candidates periodically unicast their candidacy to the BSR.
3. The BSR picks an RP set from the available candidates and periodically announces this set in a bootstrap message.
4. Bootstrap messages are flooded hop by hop throughout the domain until all routers in the domain learn the RP set.

   Note: A PIM router can receive group-to-RP mappings from either BSR or auto-RP, but not from both. Because BSR and auto-RP use different mapping algorithms, the mechanisms cannot coexist.

   Note: Static-override is configured using the override switch in the ip pim rp-address command.

   Caution: The maps distributed by BSR and Auto-RP for the same RP announcements may be different. Because the Auto-RP mapping agent resolves mapping conflicts, a PIM router that applies the BSR mapping algorithm to an Auto-RP distributed map should produce the same result as a router that applies the Auto-RP mapping algorithm. However, the reverse is not true. A PIM router that applies the BSR mapping algorithm to a BSR distributed map may produce a different result to a router that applies the Auto-RP mapping algorithm. This means that a PIM IPv4 domain can operate either BSR or Auto-RP.

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