Design Reference

The hash algorithm works as follows:
1. For each C-RP 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 C-RP C(i). The expression (G&M) guarantees that blocks of group addresses
hash to the same value for each C-RP, 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 C-RP. Thus, the block of eight addresses
are assigned to the same RP.
2. The C-RP with the highest resulting hash value is chosen as the RP for the group. In the
event of a tie, the C-RP with the highest IP address is chosen.
This algorithm runs independently on all PIM-SM routers so that every router has a
consistent view of the group-to-RP mappings.
Candidate RP considerations
The C-RP priority parameter determines 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 C-RP with the highest RP IP address is chosen as the active RP.
You cannot configure the C-RP priority. Each RP has a default C-RP 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 C-RP 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.
PIM-SM receivers and VLANs
Some designs cause unnecessary traffic flow on links in a PIM-SM domain. In these cases, traffic is
not duplicated to the receivers, but wastes bandwidth.
The following figure shows such a situation. Switch B is the DR between switches A and B. Switch C
is the RP. A receiver R is on the VLAN (V1) that connects switches A and B. A source sends
multicast data to the receiver.
IP multicast network design
142 Network Design Reference for Avaya VSP 4000 Series June 2015
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