Specifying When The Router Switches To The Sp Tree - HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual

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Specifying When the Router Switches to the SP Tree

After a source registers with the RP, the RP builds an SP tree to the source.
The RP can then distribute multicast traffic from this source to routers in the
RP tree.
Edge routers join an RP tree so that they can receive and forward multicast
packets as soon as a source begins transmitting them. After an edge router
receives a multicast packet for a group in which it is active, it too can join the
SP tree. The router looks up the source's address in the packet's header and
determines whether it has a better route to that source than the one through
the RP. If the router does find a better route in its unicast routing table, it joins
the SP tree by sending a source-specific join out the forwarding interface for
that route.
By default, an edge router begins to switch to an SP tree after it receives one
multicast packet from a source. Quickly switching to an SP tree is particularly
important for high-rate multicast applications. The SP tree reduces delay and
minimizes network congestion.
If your network primarily experiences lower-rate multicasts or brief multicast
spurts rather than sustained, high-rate multicasting, you may want to raise the
threshold so that the router does not toggle between the RP and SP tree.
To change how many packets the router must receive from a multicast source
in order to trigger the switch to an SP tree, move to the PIM sparse configu-
ration mode context and enter:
Syntax: spt-threshold <packets>
You can enter a value between 1 (the default) and 4294967295 for the number
of packets.
The SPT threshold only applies to edge routers switching to the SP tree. The
RP must generate an SP tree to a registered source, and any SPT threshold
that you set does not apply to this tree.
The PIM-SM protocol automatically manages the transition to the SP tree,
keeping the RP tree active until convergence is complete. For more informa-
tion on this process, see "Switching from an RP to an SP Tree" on page 11-9.
Configuring Multicast Support with PIM-SM
Configuring PIM-SM
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