Auto-RP and BSR
Sparse mode requires an RP for a meeting point in the network for all multicast traffic. This can be manually configured, but that becomes tedious when dealing with many devices. This is where Auto-RP and BSR become useful. Auto-RP is a Cisco proprietary protocol, and it involves two roles: the candidate RP and the mapping agent. A candidate RP is a router that announces itself as willing to act as the RP for the network. It does this by sending RP announcement packets to the 224.0.1.39 multicast address. The mapping agent listens for these RP announcement messages, compiles a list of candidate RPs, and then selects one RP. It announces the selected RP to the rest of the network using RP mapping packets sent to the 224.0.1.40 multicast address. If there are multiple candidate RPs, the mapping agent will choose the one with the highest IP address.
This brings up an important question. Since sparse mode only forwards multicast traffic when a router requests it, how do routers receive the RP announcement and RP mapping packets in the first place? That is where the Auto-RP listener feature becomes essential. When enabled on a router, Auto-RP listener uses dense mode forwarding, but only for the 224.0.1.39 and 224.0.1.40 multicast addresses. This limited dense mode behavior ensures that all routers can receive the necessary Auto-RP messages even though the network is otherwise operating in sparse mode.
BSR, or Bootstrap Router, is another method used to automatically discover the RP in a multicast network. Unlike Auto-RP, BSR is not Cisco proprietary and is part of the PIMv2 standard. Like Auto-RP, BSR has two main roles: candidate RP and candidate BSR. Candidate RPs advertise themselves as potential RPs. A candidate BSR collects this information and distributes it throughout the network.
BSR messages are sent hop by hop to the 224.0.0.13 multicast address with a TTL of 1. When a multicast router receives a BSR message, it performs a reverse path forwarding check based on the source address of the BSR to prevent routing loops. It then forwards the message out of all other PIM-enabled interfaces. One of the key differences between BSR and Auto-RP is that BSR does not require dense mode to flood its messages across the network. It uses sparse mode entirely, making it more scalable and standards-compliant.
Only one BSR can be active in the network at a time. BSR routers listen to messages from other BSRs and use a selection process based on the longest match of the multicast group. For example, if one RP wants to serve the 239.0.0.0/8 group and another wants to serve the 239.1.1.0/24 group, the second is chosen because it has a longer prefix match. If the prefix lengths are equal, then the router with the higher RP priority is selected. If priorities are the same, the router with the highest IP address becomes the active BSR.