Pim And Shortest Path Tree Switchover - Avaya 8800 Planning And Engineering, Network Design

Multicast network design
Figure 93: Example 2

PIM and Shortest Path Tree switchover

When an IGMP receiver joins a multicast group, it first joins the shared tree. Once the first packet is
received on the shared tree, the router uses the source address information in the packet to
immediately switch over to the shortest path tree (SPT).
To guarantee a simple, yet high-performance implementation of PIM-SM, the switch does not
support a threshold bit rate in relation to SPT switchover. Intermediate routers (that is, not directly
connected IGMP hosts) do not switch over to the SPT until directed to do so by the leaf routers.
Other vendors may offer a configurable threshold, such as a certain bit rate at which the SPT
switch-over occurs. Regardless of their implementation, no interoperability issues with the Avaya
Ethernet Routing Switch 8800/8600 result. Switching to and from the shared and shortest path trees
is independently controlled by each downstream router. Upstream routers relay Joins and Prunes
upstream hop-by-hop, building the desired tree as they go. Because any PIM-SM compatible router
already supports shared and shortest path trees, no compatibility issues should arise from the
implementation of configurable switchover thresholds.
June 2016 Planning and Engineering — Network Design
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