HP 3500YL - Manual (Page 120)

PIM-SM (Sparse Mode)
PIM-SM Operation and Router Types
Candidate-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 candidate RPs can also be configured with overlap­
ping or identical support.)
A candidate RP’s group-prefix configuration identifies the multicast groups
the RP is enabled to support.
If multiple candidate 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:
1. The C-RP configured with the longest group-prefix mask applicable to the
multicast group is selected to support the group. If multiple RP candidates
meet this criterion, then step 2 applies.
2. The C-RP configured with the highest priority is selected. If multiple RP
candidates meet this criterion, then step 3 applies.
3. 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. If the hash function matches
the longest mask length to multiple RP candidates, then step 4 applies.
4. The C-RP having the highest IP address is selected to support the group.
N o t e s 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 multi­
cast groups in the blocks assigned within a given domain is determined by the
bsr-candidate hash-mask-length value (range = 1 - 32; page 4-36) 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.
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