Configuring IP Multicast Protocols
December 2005
© Foundry Networks, Inc.
19 - 7
In Figure 19.1, the root node (R1) is forwarding multicast packets for group 229.225.0.1, which 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
As multicast packets reach these leaf routers, the routers check their IGMP databases for the group. If the group
is not in a router’s IGMP database, the router discards the packet and sends a prune message to the upstream
router. The router that discarded the packet also maintains the prune state for the source, group (S,G) pair. The
branch is then pruned (removed) from the multicast tree. No further multicast packets for that specific (S,G) pair
will be received from that upstream router until the prune state expires. You can configure the PIM Prune Timer
(the length of time that a prune state is considered valid).
For example, in Figure 19.1 the sender with address 207.95.5.1 is sending multicast packets to the group
229.225.0.1. If a PIM router receives any groups other than that group, the router discards the group and sends a
prune message to the upstream PIM router.
In Figure 19.2, Router R5 is a leaf node with no group members in its IGMP database. Therefore, the router must
be pruned from the multicast tree. R5 sends a prune message upstream to its neighbor router R4 to remove itself
from the multicast delivery tree and install a prune state, as seen in Figure 19.2. Router 5 will not receive any
further multicast traffic until the prune age interval expires.
When a node on the multicast delivery tree has all of its downstream branches (downstream interfaces) in the
prune state, a prune message is sent upstream. In the case of R4, if both R5 and R6 are in a prune state at the
same time, R4 becomes a leaf node with no downstream interfaces and sends a prune message to R1. With R4
in a prune state, the resulting multicast delivery tree would consist only of leaf nodes R2 and R3.
Figure 19.1
Transmission of multicast packets from the source to host group members
Group
Member
Group
Member
Leaf Node
(No Group Members)
R5
R3
R4
R6
R1
R2
Leaf Node
Leaf Node
Video Conferencing
Server
(207.95.5.1, 229.225.0.1)
(Source, Group)
...
Group
Member
Group
Member
Group
Member
...
Group
Member
Group
Member
Group
Member
...
229.225.0.1
229.225.0.1
229.225.0.1
Interrmediate Node
(No Group Members)
Содержание FastIron Edge Switch X424
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