Overview
This topic explains how Boot Strap Routers discover and announce mapping information to ensure seamless multicast traffic flow. It outlines the operational workflow for electing a primary router and establishing distribution trees within the network.
Summary
PIM uses a BSR to discover and announce RP-set information for each group prefix to all the routers in a PIM domain. This is the same function performed by Auto-RP, but a BSR is part of the PIM version 2 specification.
Cisco Auto-RP cannot co-exist with PIM BSR. Cisco Auto-RP mode must be disabled with spt-only mode.
Workflow
To avoid a single point of failure, you can configure several candidate BSRs in a PIM domain. A BSR is automatically selected from among the candidate BSRs. Bootstrap messages are used to determine which BSR has the highest priority. The router with the highest priority announces to all PIM routers in the domain that it is the BSR. Any router in the network can serve as a BSR candidate.
The elected BSR receives candidate-RP messages from all the candidate RPs in the domain. The bootstrap message sent by a BSR includes information about all of the candidate RPs.
An RP acts as the meeting place for sources and receivers of multicast data. In a PIM-SM network, sources must send their traffic to the RP. This traffic is then forwarded to receivers down a shared distribution tree. By default, when the receiver's first hop router learns about the source, it sends a join message directly to the source and creates a source-based distribution tree between the source and receiver. This source tree includes an RP only if the RP is within the shortest path between the source and receiver.
For a BSR to work for any multicast stream that spans across Cisco Catalyst SD-WAN sites, SPT-only mode is mandatory. For a BSR within a local-site multicast stream within a Cisco Catalyst SD-WAN site, it is not necessary to enable SPT-only mode.
If you have two Cisco IOS XE Catalyst SD-WAN devices in the same site, every Cisco IOS XE Catalyst SD-WAN device needs to be configured as a replicator for traffic to flow.
