Operation Manual – MulticastH3C S5500-EI Series Ethernet Switches Chapter 7 PIM Configuration7-217.3.4 Configuring a BSRNote:The BSR is dynamically elected from a number of C-BSRs. Because it is unpredictablewhich router will finally win a BSR election, the commands introduced in this sectionmust be configured on all C-BSRs.About the Hash mask length and C-BSR priority for RP selection calculation:z You can configure these parameters at three levels: global configuration level,global scope level, and BSR admin-scope level.z By default, the global scope parameters and BSR admin-scope parameters arethose configured at the global configuration level.z Parameters configured at the global scope level or BSR admin-scope level havehigher priority than those configured at the global configuration level.I. Performing basic C-BSR configurationA PIM-SM domain can have only one BSR, but must have at least one C-BSR. Anyrouter can be configured as a C-BSR. Elected from C-BSRs, a BSR is responsible forcollecting and advertising RP information in the PIM-SM.C-BSRs should be configured on routers in the backbone network. When configuring arouter as a C-BSR, make sure that router is PIM-SM enabled. The BSR electionprocess is as follows:z Initially, every C-BSR assumes itself to be the BSR of this PIM-SM domain, anduses its interface IP address as the BSR address to send bootstrap messages.z When a C-BSR receives the bootstrap message of another C-BSR, it firstcompares its own priority with the other C-BSR’s priority carried in the message.The C-BSR with a higher priority wins. If there is a tie in the priority, the C-BSRwith a higher IP address wins. The loser uses the winner’s BSR address to replaceits own BSR address and no longer assumes itself to be the BSR, while the winnerkeeps its own BSR address and continues assuming itself to be the BSR.Configuring a legal range of BSR addresses enables filtering of BSR messages basedon the address range, thus to prevent malicious hosts from initiating attacks bydisguising themselves as legitimate BSRs. To protect legitimate BSRs from beingmaliciously replaced, preventive measures are taken specific to the following twosituations:1) Some malicious hosts intend to fool routers by forging BSR messages and changethe RP mapping relationship. Such attacks often occur on border routers. Becausea BSR is inside the network whereas hosts are outside the network, you canprotect a BSR against attacks from external hosts by enabling border routers to
