3Com Switch 8800 Advanced Software V5 Configuration Guide

Configuring IPv6 PIM Common Information 693
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■ Generally, a smaller distance from the filter to the IPv6 multicast source results
in a more remarkable filtering effect.
■ This filter works not only on independent IPv6 multicast data but also on IPv6
multicast data encapsulated in register messages.
Configuring IPv6 PIM
Hello Options
No matter in an IPv6 PIM-DM domain or an IPv6 PIM-SM domain, the hello
messages sent among devices contain many configurable options, including:
■ DR_Priority (for IPv6 PIM-SM only): priority for DR election. The higher the
priority is, the easier it is for the device to win DR election. You can configure
this parameter on all the devices in a multi-access network directly connected
to IPv6 multicast sources or receivers.
■ Holdtime: the timeout time of IPv6 PIM neighbor reachability state. When this
timer times out, if the device has received no hello message from an IPv6 PIM
neighbor, it assumes that this neighbor has expired or become unreachable.
You can configure this parameter on all devices in the IPv6 PIM domain. If you
configure different values for this timer on different IPv6 PIM neighbors, 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 IPv6 PIM domain. If different LAN-delay or override-interval values result
from the negotiation among all the IPv6 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 IPv6 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 IPv6
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 an IPv6 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 an IPv6 PIM device does not change unless the
status of the device changes (for example, when IPv6 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 an IPv6 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