KB.15.15

VRRP: PIM-SM is fully interoperable with VRRP to quickly transition

multicast routes in the event of a failover.

MIB support on the switches covered

in this guide:

PIM-SM supports the Protocol Independent Multicast MIB for

IPv4 (RFC 2934.)

With some exceptions, PIM-SM supports the parts of the

multicast routing MIB (RFC 2932) applicable to PIM-SM

operation.

PIM draft specifications: Compatible with PIM-SM specification ( RFC 4061.)

BSR implementation: Complies with RFC 5059 (scope zones are not supported.)

PIM-SM operation and router types

Unlike PIM-DM, PIM-SM assumes that most hosts do not want to receive multicast traffic, and uses

a non-flooding multicast model to direct traffic for a particular multicast group from the source to

the VLAN(s) where there are multicast receivers that have joined the group. As a result, this model

sends traffic only to the routers that specifically request it.

Pim-SM operation

In a given PIM-SM domain, routers identified as DRs, RPs, and a BSR participate in delivering

multicast traffic to the IP multicast receivers that request it. This approach avoids the flooding method

of distributing multicast traffic (employed by PIM-DM) and is best suited for lower bandwidth

situations.

The software supports the following operation to enable multicast traffic delivery within a PIM-SM

domain:

• From a pool of eligible DR candidates in each VLAN, one DR is elected for each VLAN interface

having at least one PIM-SM router. In a multinetted domain, this DR supports multicast traffic

from a source on any subnet in the VLAN.

• From a pool of eligible BSR candidates in the domain, one BSR is elected for the entire domain.

• From a pool of eligible C-RPs, one is elected to support each multicast group or range of

groups allowed in the domain, excluding any group supported only by static RPs. The multicast

groups allowed in the domain are determined by the aggregation of the groups allowed by

the individually configured RPs and any static RPs. (Note that RP-Cs and static RP’s can be

configured with overlapping support for a given set of multicast groups.)

Rendezvous-point tree (RPT)

When a DR in a VLAN receives traffic for a particular multicast group from a source on that VLAN,

the DR encapsulates the traffic and forwards it to the RP elected to support that multicast group.

The RP decapsulates the traffic and forwards it on toward the multicast receiver(s) requesting that

group. This forms an RPT extending from the DR through any intermediate PIM-SM routers leading

to the PIM-SM edge router(s) for the multicast receiver(s) requesting the traffic. (If the RP has no

current join requests for the group, the traffic is dropped at the RP.)

94 PIM-SM (Sparse Mode)