BSR (Boot Strap Router) step by step

BSR Boot Strap Router

1. Candidate BSR sends multicast message to ALL Pim routers (PIM2 type4 message) to announce it is the candidate BSR, this is forwarded hop by hop as long as it passes RPF check
2. After best BSR is elected from step 1 C-BSR, the Candidate RPs announce themselves via Unicast RP-Announce message (PIMv2 type8 message) to the IP of the BSR
3. BSR will then flood C-RP along with hash value for PIM routers to choose the RP for each group (Now ALL PIM routers know the BSR, C-RPs and their groups. According to the hash-value advertised by the BSR)

links to detailed information and packet captures: