Dell BigIron RX Series, Руководство по настройке

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648 BigIron RX Series Configuration Guide
53-1002253-01
DVMRP overview
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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 96 . 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 96 , 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 (subnets 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 97 , 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.