Static RP (static RP):
This option forwards traffic in the same way as an RP, but requires manual configuration on all routers in the
domain to be effective.
All of the above functions can be enabled on each of several routers in a PIMSM domain.
DR
In a VLAN populated by one or more routers running PIM-SM, one such router is elected the DR for that VLAN.
When the DR receives a Join request from a multicast receiver on that VLAN, it forwards the join toward the
router operating as the RP for the requested multicast group.
Where multiple PIM-SM routers exist in a VLAN, the following criteria is used to elect a DR:
Procedure
1.
The router configured with the highest DR priority in the VLAN is elected.
2.
If multiple routers in the VLAN are configured with the highest DR priority, 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, all routers on a VLAN need to 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 zero (0.)
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 BSR function enables this operation by doing the
following:
Procedure
1.
Learns the group-to-RP mappings on the 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.
NOTE:
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.)
Chapter 4 PIM-SM (Sparse Mode)
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