KB.15.15

that candidate is selected to support the group. Apply step 4 of this procedure if the hash

function matches the longest mask length to multiple RP candidates.

4. The C-RP having the highest IP address is selected to support the group.

NOTE: In a PIM-SM domain where there are overlapping ranges of multicast groups configured

on the C-RPs, discrete ranges of these groups are assigned to the domain's C-RPs in blocks of

sequential group numbers. The number of multicast groups in the blocks assigned within a given

domain is determined by the bsr-candidate hash-mask-length value (range=1 to 32)

configured on the elected BSR for the domain. A higher value means fewer sequential group

numbers in each block of sequential group numbers, which results in a wider dispersal of multicast

groups across the C-RPs in the domain.

As indicated above, multiple C-RPs can be configured to support the same multicast group(s.) This

is the generally recommended practice and results in redundancy that helps to prevent loss of

support for desired multicast groups in the event that a router in the domain becomes unavailable.

Configuring a C-RP to support a given multicast group does not ensure election of the C-RP to

support that group unless the group is excluded from all other RPs in the domain.

Also, within a PIM-SM domain, a router can be configured as a C-RP available for a given multicast

group or range of groups and as the static RP for a given multicast group or range of groups. The

recommended practice is to use C-RPs for all multicast groups unless there is a need to ensure that

a specific group or range of groups is always supported by the same routing switch. See “Static

RP (static RP)” (page 99).

Redundant Group Coverage Provides Fault-Tolerance

If a C-RP elected to support a particular multicast group or range of groups becomes unavailable,

the router is excluded from the RP-set. If the multicast group configuration of one or more other

C-RPs overlaps the configuration in the failed RP, then another C-RP is elected to support the multicast

group(s) formerly relying on the failed RP.

Static RP (static RP)

General application

Like C-RPs, static RPs control multicast forwarding of specific multicast groups or ranges of contiguous

groups. However, static RPs are not dynamically learned, and increase the configuration and

monitoring effort needed to maintain them. As a result, static RPs are not generally recommended

for use except where one of the following conditions applies:

• It is desirable to designate a specific router interface as a backup RP for specific group(s.)

• Specific multicast groups are expected, and a static RP would help to avoid overloading a

given RP with a high volume of multicast traffic.

• A C-RP for the same group(s) is less reliable than another RP that would not normally be elected

to support the group(s.)

• Tighter traffic control or a higher priority is desired for specific multicast groups

Pim-SM router types 99