PIM-SM Device Types
Devices configured with PIM-SM interfaces can fill one or more roles.
- Bootstrap router (BSR)
- A router that distributes
rendezvous point (RP) information to the other PIM-SM devices in the domain.
Each PIM-SM domain has one active BSR. For redundancy, you can configure ports
on multiple devices as candidate BSRs (C-BSRs). PIM-SM uses an election process
to select one C-BSR as the BSR for the domain. The BSR with the highest BSR
priority (a user-configurable parameter) is elected. If the priorities result in
a tie, then the C-BSR interface with the highest IP address is elected.
- The BSR must be configured as
part of the Layer 3 core network.
- For more information, see Bootstrap Router Protocol.
- Rendezvous point (RP)
- The meeting point for PIM-SM
sources and receivers. A PIM-SM domain can have multiple RPs, but each PIM-SM
multicast group address can have only one active RP. PIM-SM devices learn the
addresses of RPs and the groups for which they are responsible from messages
that the BSR sends to each PIM-SM device.
- The RP must be configured as part
of the Layer 3 core network.
- A best practice is to configure
the same ports as C-BSRs and RPs.
- Designated router (DR)
- Hosts and routers send Join
messages to the RP for the group in which they are interested. The local network
selects one of its upstream routers as the DR. All hosts below a DR send IGMP
Join messages to the DR. The DR sends only one Join message to the RP on behalf
of all its interested hosts. The RP receives the first few packets of the
multicast stream, encapsulated in the PIM register message, from the source
hosts. These messages are sent as a unicast to the RP. The RP decapsulates these
packets and forwards them to the respective DRs.
- DR election is based first on the
router with the highest configured DR priority for an interface and next on the
router with the highest IP address. You can use the ip pim dr-priority
command to configure DR priority.
