Avaya 8800 Planning And Engineering, Network Design page 210

Multicast network design
The hash algorithm works as follows:
1. For each CRP router with matching group address ranges, a hash value is calculated
according to the formula:
Hash value [G, M, C(i)] = {1 103 515 245 * [(1 103 515245 * (G&M) +12 345) XOR C(i)] + 12
345} mod 2^31
The hash value is a function of the group address (G), the hash mask (M), and the IP
address of the CRP C(i). The expression (G&M) guarantees that blocks of group addresses
hash to the same value for each CRP, and that the size of the block is determined by the
hash mask.
For example, if the hash mask is 255.255.255.248, the group addresses 239.0.0.0 through
239.0.0.7 yield the same hash value for a given CRP. Thus, the block of eight addresses are
assigned to the same RP.
2. The CRP with the highest resulting hash value is chosen as the RP for the group. In the
event of a tie, the CRP with the highest IP address is chosen.
This algorithm is run independently on all PIM-SM routers so that every router has a
consistent view of the group-to-RP mappings.
Candidate RP considerations
The CRP priority parameter helps to determine an active RP for a group. The hash values for
different RPs are only compared for RPs with the highest priority. Among the RPs with the highest
priority value and the same hash value, the CRP with the highest RP IP address is chosen as the
active RP.
You cannot configure the CRP priority. Each RP has a default CRP priority value of 0, and the
algorithm uses the RP if the group address maps to the grp-prefix that you configure for that RP. If a
different router in the network has a CRP priority value greater than 0, the switch uses this part of
the algorithm in the RP election process.
Currently, you cannot configure the hash mask used in the hash algorithm. Unless you configure a
different PIM BSR in the network with a nondefault hash mask value, the default hash mask of
255.255.255.252 is used. Static RP configurations do not use the BSR hash mask; they use the
default hash mask.
For example:
RP1 = 128.10.0.54 and RP2 = 128.10.0.56. The group prefix for both RPs is 238.0.0.0/255.0.0.0.
Hash mask = 255.255.255.252.
The hash function assigns the groups to RPs in the following manner:
The group range 238.1.1.40 to 238.1.1.51 (12 consecutive groups) maps to 128.10.0.56. The group
range 238.1.1.52 to 238.1.1.55 (4 consecutive groups) maps to 128.10.0.54. The group range
238.1.1.56 to 238.1.1.63 (8 consecutive groups) maps to 128.10.0.56.
June 2016 Planning and Engineering — Network Design
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