Avaya 8800 Planning And Engineering, Network Design page 209

PIM-SM design and the BSR hash algorithm
To optimize the flow of traffic down the shared trees in a network that uses bootstrap router (BSR) to
dynamically advertise candidate RPs, consider the hash function. The hash function used by the
BSR to assign multicast group addresses to each candidate RP (CRP).
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 candidate 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 99: 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.
• 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 CRPs.
• 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 CRPs take over for a particular group address in the event of
primary RP failure.
June 2016
Protocol Independent Multicast-Sparse Mode guidelines
Planning and Engineering — Network Design
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