Operation Manual – MulticastH3C S5600 Series Ethernet Switches Chapter 4 PIM Configuration4-81) Routers on the multi-access network send hello messages to one another. Thehello messages contain the router priority for DR election. The router with thehighest DR priority will become the DR.2) In the case of a tie in the router priority, or if any router in the network does notsupport carrying the DR-election priority in hello messages, the router with thehighest IP address will win the DR election.When the DR fails, a timeout in receiving hello message triggers a new DR electionprocess among the other routers.Note:z S5600 series Ethernet switches do not support DR priority. DR election is based onIP addresses.z In a PIM-DM domain, a DR serves as an IGMPv1 querier.III. RP discoveryThe RP is the core of a PIM-SM domain. For a small-sized, simple network, one RP isenough for forwarding information throughout the network, and the position of the RPcan be statically specified on each router in the PIM-SM domain. In most cases,however, a PIM-SM network covers a wide area and a huge amount of multicast trafficneeds to be forwarded through the RP. To lessen the RP burden and optimize thetopological structure of the RPT, each multicast group should have its own RP.Therefore, a bootstrap mechanism is needed for dynamic RP election. For this purpose,a bootstrap router (BSR) should be configured.As the administrative core of a PIM-SM domain, the BSR collects advertisementmessages (C-RP-Adv messages) from candidate-RPs (C-RPs) and chooses theappropriate C-RP information for each multicast group to form an RP-Set, which is adatabase of mappings between multicast groups and RPs. The BSR then floods theRP-Set to the entire PIM-SM domain. Based on the information in these RP-Sets, allrouters (including the DRs) in the network can calculate the location of thecorresponding RPs.A PIM-SM domain (or an administratively scoped region) can have only one BSR, butcan have multiple candidate-BSRs (C-BSRs). Once the BSR fails, a new BSR isautomatically elected from the C-BSRs through the bootstrap mechanism to avoidservice interruption. Similarly, multiple C-RPs can be configured in a PIM-SM domain,and the position of the RP corresponding to each multicast group is calculated throughthe BSR mechanism.Figure 4-4 shows the positions of C-RPs and the BSR in the network.
