Hello Timer; Override And Propagation Delay Timers - HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual

Configuring Multicast Support with PIM-SM
Configuring PIM-SM

Hello Timer

Routers transmit periodic hellos through PIM interfaces to signal that the
connection is still active. The hello-timer option determines how often an
interface sends a hello. The router also uses this setting to compute the hello
holdtime, which it includes in hello packets to instruct neighbors how long to
wait for the next hello before removing the connection from any outgoing
interface lists. (The hello holdtime should be three and a half times the hello
timer to allow for the occasional lost packet.) Shortening the hello timer
indirectly reduces the hello holdtime, which means that upstream routers will
more quickly stop sending unnecessary multicasts.
The neighbor timeout (nbr-timeout option) is the maximum amount of time
the router itself will wait for a hello message from a neighbor before consid-
ering that neighbor unreachable. You should generally configure the neighbor
timeout to three and a half times the neighbor's hello timer. The neighbor
timeout configured on the interface takes precedence over the hello holdtime
included in the neighbor's hello.

Override and Propagation Delay Timers

The override and propagation delay timers deal with the join that a router
sends to override a prune sent by another router in its multi-access network.
For example, a network may have multiple routers serving VLAN 10. One of
the routers on VLAN 10 receives multicast traffic that it does not need, so it
sends a prune to the upstream neighbor. However, other routers in VLAN 10
may still need the traffic. When a router receives a prune for a group on that
group's incoming interface, it knows that another router in the subnet is
attempting to prune the connection. If the router still needs the traffic, it
schedules a triggered join to the upstream neighbor to prevent the transmis-
sion from being disrupted.
The override delay interval (override-interval option) is the maximum
amount of time that the router will wait before sending its join. Because other
routers in the multi-access network may also sends joins, the router selects a
random time within the interval so that the upstream neighbor is not flooded
with all the joins at the same time. The propagation-delay option sets the
amount of time that the router expects the join will take to reach the neighbor.
When a router receives a prune for a group on an interface that connects to a
multi-access network, it schedules that interface for deletion from the appro-
priate outgoing interface list. The router does not immediately prune the
interface so that other routers in the multi-access network can have a chance
to override the prune. The period that the router waits for overrides before
11-39