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Configuring DVMRP Configuring DVMRP
OmniSwitch 6800/6850/9000 Advanced Routing Configuration Guide December 2007 page 6-15
Pruning
DVMRP uses a flood-and-prune mechanism that starts by delivering multicast traffic to all routers in the
network. This means that, initially, traffic is flooded down a multicast delivery tree. DVMRP routers then
prune this flow where the traffic is unwanted. Routers that have no use for the traffic send DVMRP Prune
messages up the delivery tree to stop the flow of unwanted multicast traffic, thus pruning the unwanted
branches of the tree. After pruning, a source distribution tree for that specific source exists.
However, the source distribution tree that results from DVMRP pruning reverts back to the original deliv-
ery tree when the prunes time out. When a prune times out, traffic is again flooded down the branch.
The ip dvmrp prune-lifetime command sets the period of time that a prune will be in effect — essen-
tially, the prune’s lifetime. When the prune-lifetime period expires, the interface is joined back onto the
multicast delivery tree. (If unwanted multicast traffic continues to arrive at the interface, the prune mecha-
nism is reinitiated and the cycle continues.) For example, the following command sets a prune’s lifetime to
7200 seconds:
-> ip dvmrp prune-lifetime 7200
Refer to “More About Prunes” below for further information on the ip dvmrp prune-lifetime command
and how it affects the lifetime of prunes sent and, in some cases, received.
The ip dvmrp prune-timeout command sets the Prune packet retransmission interval. This is the dura-
tion of time that the router will wait before retransmitting a Prune message if it continues to receive
unwanted multicast traffic. For example, the following command sets the Prune packet retransmission
interval to forty seconds:
-> ip dvmrp prune-timeout 40
Note. Current global DVMRP parameter values—including the ip dvmrp prune-lifetime value and the
ip dvmrp prune-timeout value—can be viewed via the show ip dvmrp command. Current DVMRP
prunes can be viewed via the show ip dvmrp prune command.
More About Prunes
Prune-Lifetime Values in Sent Prune Packets
The value of ip dvmrp prune-lifetime is set to 7200 seconds (two hours) by default. On leaf routers (that
is, routers that have no further downstream dependent routers), the value of ip dvmrp prune-lifetime is
inserted into prune packets sent upstream as their lifetime value.
However, when a branch router (that is, a router that does have further downstream dependent routers)
sends a prune upstream, the prune-lifetime value inserted into the prune packet is the smallest of the
following values:
• the value of ip dvmrp prune-lifetime on the sending device
• the amount of lifetime that remains for each individual prune on the router’s timer queue that was
received for the pruned group. (When a prune is queued on the router’s timer queue, its lifetime value
decrements until the prune expires.)