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Configuring IP Multicast Protocols
December 2005
© Foundry Networks, Inc.
19 - 19
Changing the Shortest Path Tree (SPT) Threshold
In a typical PIM Sparse domain, there may be two or more paths from a DR (designated router) for a multicast
source to a PIM group receiver.
•
Path through the RP – This is the path the Layer 3 Switch uses the first time it receives traffic for a PIM group.
However, the path through the RP may not be the shortest path from the Layer 3 Switch to the receiver.
•
Shortest Path – Each PIM Sparse router that is a DR for a multicast source calculates a shortest path tree
(SPT) to all the PIM Sparse group receivers within the domain, with the Layer 3 Switch itself as the root of the
tree. The first time a Foundry Layer 3 Switch configured as a PIM router receives a packet for a PIM receiver,
the Layer 3 Switch sends the packet to the RP for the group. The Layer 3 Switch also calculates the SPT from
itself to the receiver. The next time the Layer 3 Switch receives a PIM Sparse packet for the receiver, the
Layer 3 Switch sends the packet toward the receiver using the shortest route, which may not pass through the
RP.
By default, the device switches from the RP to the SPT after receiving the first packet for a given PIM Sparse
group. The Layer 3 Switch maintains a separate counter for each PIM Sparse source-group pair.
After the Layer 3 Switch receives a packet for a given source-group pair, the Layer 3 Switch starts a PIM data timer
for that source-group pair. If the Layer 3 Switch does not receive another packet for the source-group pair before
the timer expires, it reverts to using the RP for the next packet received for the source-group pair. In accordance
with the PIM Sparse RFC’s recommendation, the timer is 210 seconds and is not configurable. The counter is
reset to zero each time the Layer 3 Switch receives a packet for the source-group pair.
You can change the number of packets that the Layer 3 Switch sends using the RP before switching to using the
SPT. To do so, use the following CLI method.
FastIron SuperX Router(config)# router pim
FastIron SuperX Router(config-pim-router)# spt-threshold 1000
Syntax:
[no] spt-threshold infinity | <num>
The
infinity
| <num> parameter specifies the number of packets. If you specify
infinity
, the Layer 3 Switch sends
packets using the RP indefinitely and does not switch over to the SPT. If you enter a specific number of packets,
the Layer 3 Switch does not switch over to using the SPT until it has sent the number of packets you specify using
the RP.
Changing the PIM Join and Prune Message Interval
By default, the Layer 3 Switch sends PIM Sparse Join/Prune messages every 60 seconds. These messages
inform other PIM Sparse routers about clients who want to become receivers (Join) or stop being receivers
(Prune) for PIM Sparse groups.
You can change the Join/Prune message interval using the following CLI method.
NOTE:
Use the same Join/Prune message interval on all the PIM Sparse routers in the PIM Sparse domain. If
the routers do not all use the same timer interval, the performance of PIM Sparse can be adversely affected.
To change the Join/Prune interval, enter commands such as the following:
FastIron SuperX Router(config)# router pim
FastIron SuperX Router(config-pim-router)# message-interval 30
Syntax:
[no] message-interval <num>
The <num> parameter specifies the number of seconds and can from 1 – 65535. The default is 60.
Dropping PIM Traffic in Hardware
Unwanted PIM Dense or PIM Sparse multicast traffic can be dropped in hardware on Layer 3 Switches. This
feature does not apply to DVMRP traffic. Refer to “Dropping PIM Traffic in Hardware” on page 19-31.
Summary of Contents for FastIron Edge Switch X424
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