H3C Technologies H3C S7500E Series Switches - Manual (Page 175)

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With the hello message filter configured, if hello messages of an existing PIM neighbor fail to
pass the filter, the PIM neighbor will be removed automatically when it times out.
Configuring PIM Hello Options
No matter in a PIM-DM domain or a PIM-SM domain, the hello messages sent among routers
contain many configurable options, including:
z DR_Priority (for PIM-SM only): priority for DR election. The device with the highest priority
wins the DR election. You can configure this parameter on all the routers in a multi-access
network directly connected to multicast sources or receivers.
z Holdtime: the timeout time of PIM neighbor reachability state. When this timer times out, if
the router has received no hello message from a neighbor, it assumes that this neighbor
has expired or become unreachable.
z LAN_Prune_Delay: the delay of prune messages on a multi-access network. This option
consists of LAN-delay (namely, prune message delay), override-interval, and neighbor
tracking flag. If the LAN-delay or override-interval values of different PIM routers on a
multi-access subnet are different, the largest value will take effect. If you want to enable
neighbor tracking, the neighbor tracking feature should be enabled on all PIM routers on a
multi-access subnet.
The LAN-delay setting will cause the upstream routers to delay processing received prune
messages.
The override-interval sets the length of time a downstream router is allowed to wait before
sending a prune override message. When a router receives a prune message from a
downstream router, it does not perform the prune action immediately; instead, it maintains the
current forwarding state for a period of LAN-delay plus override-interval. If the downstream
router needs to continue receiving multicast data, it must send a prune override message within
the prune override interval; otherwise, the upstream route will perform the prune action when
the period of LAN-delay plus override-interval time out.
A hello message sent from a PIM router contains a generation ID option. The generation ID is a
random value for the interface on which the hello message is sent. Normally, the generation ID
of a PIM router does not change unless the status of the router changes (for example, when
PIM is just enabled on the interface or the device is restarted). When the router starts or
restarts sending hello messages, it generates a new generation ID. If a PIM router finds that the
generation ID in a hello message from the upstream router has changed, it assumes that the
status of the upstream neighbor is lost or the upstream neighbor has changed. In this case, it
triggers a join message for state update.
If you disable join suppression (namely, enable neighbor tracking), the join suppression feature
should be disabled on all PIM routers on a multi-access subnet; otherwise, the upstream router
will fail to explicitly track which downstream routers are joined to it.