3Com Switch 8800 Advanced Software V5 Configuration Guide

Configuring PIM Common Information 589
Configuring PIM Hello
Options
No matter in a PIM-DM domain or a PIM-SM domain, the hello messages sent
among devices contain many configurable options, including:
■ 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 devices in a multi-access network directly connected to multicast sources or
receivers.
■ Holdtime: the timeout time of PIM neighbor reachability state. When this timer
times out, if the device has received no hello message from a neighbor, it
assumes that this neighbor has expired or become unreachable. You can
configure this parameter on all devices in the PIM domain. If you configure
different values for this timer on different neighboring devices, the largest
value will take effect.
■ LAN_Prune_Delay: the delay of prune messages on a multi-access network.
This option consists of LAN-delay (namely, prune delay), override-interval, and
neighbor tracking flag bit. You can configure this parameter on all devices in
the PIM domain. If different LAN-delay or override-interval values result from
the negotiation among all the PIM devices, the largest value will take effect.
The LAN-delay setting will cause the upstream devices to delay processing received
prune messages. If the LAN-delay setting is too small, it may cause the upstream
device to stop forwarding multicast packets before a downstream device sends a
prune override message. Therefore, be cautious when configuring this parameter.
The override-interval sets the length of time a downstream device is allowed to
wait before sending a prune override message. When a device receives a prune
message from a downstream device, it does not perform the prune action
immediately; instead, it maintains the current forwarding state for a period of time
defined by LAN-delay. If the downstream device 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
LAN-delay timer times out.
A hello message sent from a PIM device 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 device does not change unless the
status of the device changes (for example, when PIM is just enabled on the
interface or the device is restarted). When the device starts or restarts sending
hello messages, it generates a new generation ID. If a PIM device finds that the
generation ID in a hello message from the upstream device 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 upstream
device will explicitly track which downstream devices are joined to it. The join
suppression feature should be enabled or disable on all PIM devices on the same
subnet.
Configuring hello options globally
Follow these steps to configure hello options globally:
To do... Use the command... Remarks
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