Chapter 20
| Multicast Routing
Configuring PIM for IPv4
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◆ LAN Prune Delay – Causes this device to inform downstream routers of how
long it will wait before pruning a flow after receiving a prune request.
(Default: Disabled)
When other downstream routers on the same VLAN are notified that this
upstream router has received a prune request, they must send a Join to
override the prune before the prune delay expires if they want to continue
receiving the flow. The message generated by this command effectively
prompts any downstream neighbors with hosts receiving the flow to reply with
a Join message. If no join messages are received after the prune delay expires,
this router will prune the flow.
The sum of the Override Interval and Propagation Delay are used to calculate
the LAN prune delay.
◆ Override Interval – The time required for a downstream router to respond to a
LAN Prune Delay message by sending back a Join message if it wants to
continue receiving the flow referenced in the message. (Range: 500-6000
milliseconds; Default: 2500 milliseconds)
The override interval and the propagation delay are used to calculate the LAN
prune delay. If a downstream router has group members which want to
continue receiving the flow referenced in a LAN prune delay message, then the
override interval represents the time required for the downstream router to
process the message and then respond by sending a Join message back to the
upstream router to ensure that the flow is not terminated.
◆ Propagation Delay – The time required for a LAN prune delay message to
reach downstream routers. (Range: 100-5000 milliseconds; Default: 500
milliseconds)
The override interval and pro po gat ion delay are used to calculate the LAN
prune delay. If a downstream router has group members which want to
continue receiving the flow referenced in a LAN prune delay message, then the
propagation delay represents the time required for the LAN prune delay
message to be propagated down from the upstream router to all downstream
routers attached to the same VLAN interface.
◆ Trigger Hello Delay – The maximum time before transmitting a triggered PIM
Hello message after the router is rebooted or PIM is enabled on an interface.
(Range: 0-5 seconds; Default: 5 seconds)
When a router first starts or PIM is enabled on an interface, the hello delay is set
to random value between 0 and the trigger hello delay. This prevents
synchronization of Hello messages on multi-access links if multiple routers are
powered on simultaneously.
Also, if a Hello message is received from a new neighbor, the receiving router
will send its own Hello message after a random delay between 0 and the
trigger hello delay.
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