Design Reference

PIM-SM design and the BSR hash algorithm
To optimize the flow of traffic down the shared trees in a network that uses a BSR to dynamically
advertise candidate RPs, consider the hash function. The BSR uses the hash function to assign
multicast group addresses to each C-RP.
The BSR distributes the hash mask used to compute the RP assignment. For example, if two RPs
are candidates for the range 239.0.0.0 through 239.0.0.127, and the hash mask is 255.255.255.252,
that range of addresses is divided into groups of four consecutive addresses and assigned to one or
the other C-RP.
The following figure illustrates a suboptimal design where Router A sends traffic to a group address
assigned to RP D. Router B sends traffic assigned to RP C. RP C and RP D serve as backups for
each other for those group addresses. To distribute traffic, it is desirable that traffic from Router A
use RP C and that traffic from Router B use RP D.
Figure 71: Example multicast network
While still providing redundancy in the case of an RP failure, you can ensure that the optimal shared
tree is used by using the following methods.
1. Use the hash algorithm to proactively plan the group-address-to-RP assignment.
Use this information to select the multicast group address for each multicast sender on the
network and to ensure optimal traffic flows. This method is helpful for modeling more
complex redundancy and failure scenarios, where each group address has three or more C-
RPs.
2. Allow the hash algorithm to assign the blocks of addresses on the network, and then view
the results using the command show ip pim active-rp .
Use the command output to assign multicast group addresses to senders that are located
near the indicated RP. The limitation to this approach is that while you can easily determine
the current RP for a group address, the backup RP is not shown. If more than one backup
for a group address exists, the secondary RP is not obvious. In this case, use the hash
algorithm to reveal which of the remaining C-RPs take over for a particular group address in
the event of primary RP failure.
Protocol Independent Multicast-Sparse Mode guidelines
June 2015 Network Design Reference for Avaya VSP 4000 Series 141
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