N o t e
2. If multiple routers in the VLAN are configured with the highest DR priority,
then the router having the highest IP address is elected.
In a given domain, each VLAN capable of receiving multicast traffic from a
unicast source should have at least one DR. (Enabling PIM-SM on a VLAN
automatically enables the router as a DR for that VLAN.) Because there is an
election process for DR on each VLAN, it is generally recommended that all
routers on a VLAN be enabled for DR. Where it is important to ensure that a
particular router is elected as the DR for a given VLAN, you can increase the
DR priority on that VLAN configuration for that router.
If it is necessary to prevent a router from operating as a DR on a given VLAN,
disable DR operation by configuring the DR priority as 0 (zero).
Bootstrap Router (BSR)
Before a DR can forward encapsulated packets for a specific multicast group
to an RP, it must know which router in the domain is the elected RP for that
multicast group. The bootstrap router (BSR) function enables this operation
by doing the following:
1. Learns the group-to-RP mappings on the Candidate Rendezvous Points
(C-RPs) in the domain by reading the periodic advertisements each one
sends to the BSR.
2. Distributes the aggregate C-RP information as an RP-set to the PIM-SM
routers in the domain. This is followed by an election to assign a specific
multicast group or range of groups to the C-RPs in the domain. (The
software supports assignment of up to four multicast addresses and/or
ranges of multicast addresses to a C-RP.)
The BSR periodically sends bootstrap messages to the other PIM-SM routers
in the domain to maintain and update the RP-set data throughout the domain,
and to maintain its status as the elected BSR.
Where static RPs are configured in the domain to support the same multicast
group(s) as one or more (dynamic) C-RPs, then the RP-set data has the
precedence for assigning RPs for these groups unless the static-RPs have been
configured with the override option and if the multicast group mask for the
static-RP equals or exceeds the same mask for the applicable C-RP(s). Refer
to the Note on page 4-17.
BSR Configuration and Election. There should be multiple BSR candi
dates configured in a PIM-SM domain so that if the elected BSR becomes
unavailable, another router will take its place. In the BSR election process,
PIM-SM (Sparse Mode)
PIM-SM Operation and Router Types