C-RP election
Within a PIM-SM domain, different RPs support different multicast addresses or ranges of multicast addresses. (That is, a given PIM-SM multicast group or range of groups is supported by only one active RP, although other C-RPs can also be configured with overlapping or identical support.)
A C-RP's group-prefix configuration identifies the multicast groups the RP is enabled to support.
If multiple C-RPs have group-prefixes configured so that any of these RPs can support a given multicast group, then the following criteria are used to select the RP to support the group:
- The C-RP configured with the longest group-prefix mask applicable to the multicast group is selected to support the group. Step 2 of this procedure applies if multiple RP candidates meet this criterion.
- The C-RP configured with the highest priority is selected. Step 3 of this procedure applies if multiple RP candidates meet this criterion.
- A hash function (using the
configured
bsr-candidate hash-mask-length
value) generates a series of mask length values that are individually assigned to the set of eligible C-RPs. If the hash function matches a single RP candidate to a longer mask length than the other candidates, 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. - The C-RP having the highest IP address is selected to support the group.
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).