Configuring IP Multicast Protocols
December 2005
© Foundry Networks, Inc.
19 - 53
DVMRP employs
reverse path forwarding
and
pruning
to keep source specific multicast delivery trees
with the
minimum number of branches required to reach all group members. DVMRP builds a multicast tree for each
source and destination host group.
Initiating DVMRP Multicasts on a Network
Once DVMRP is enabled on each router, a network user can begin a video conference multicast from the server
on R1.
Multicast Delivery Trees
are initially formed by source-originated multicast packets that are propagated
to downstream interfaces as seen in Figure 19.7. When a multicast packet is received on a DVMRP-capable
router interface, the interface checks its DVMRP routing table to determine whether the interface that received the
message provides the shortest path back to the source. If the interface does provide the shortest path, the
interface forwards the multicast packet to adjacent peer DVMRP routers, except for the router interface that
originated the packet. Otherwise, the interface discards the multicast packet and sends a prune message back
upstream. This process is known as
reverse path
forwarding
.
In Figure 19.7, the root node (R1) is forwarding multicast packets for group 229.225.0.2 that it receives from the
server to its downstream nodes, R2, R3, and R4. Router R4 is an intermediate router with R5 and R6 as its
downstream routers. Because R5 and R6 have no downstream interfaces, they are leaf nodes.
The receivers in this example are those workstations that are resident on routers R2, R3, and R6.
Pruning a Multicast Tree
After the multicast tree is constructed,
pruning
of the tree will occur after IP multicast packets begin to traverse
the tree.
As multicast packets reach leaf networks (sub-nets with no downstream interfaces), the local IGMP database
checks for the recently arrived IP multicast packet address. If the local database does not contain the address
(the address has not been learned), the router prunes (removes) the address from the multicast tree and no longer
receives multicasts until the prune age expires.
In Figure 19.8, Router 5 is a leaf node with no group members in its local database. Consequently, Router 5
sends a prune message to its upstream router. This router will not receive any further multicast traffic until the
prune age interval expires.
Содержание FastIron Edge Switch X424
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