Configuration Guide User guide

1522 FastIron Configuration Guide
53-1002494-02
PIM Dense
Manually added groups are included in the group information displayed by the following
commands:
• show ip igmp group
• show ip pim group
PIM Dense
NOTE
This section describes the “dense” mode of PIM, described in RFC 1075. Refer to “PIM Sparse” on
page 1531 for information about PIM Sparse.
PIM was introduced to simplify some of the complexity of the routing protocol at the cost of
additional overhead tied with a greater replication of forwarded multicast packets. PIM builds
source-routed multicast delivery trees and employs reverse path check when forwarding multicast
packets.
There are two modes in which PIM operates: Dense and Sparse. The Dense Mode is suitable for
densely populated multicast groups, primarily in the LAN environment. The Sparse Mode is
suitable for sparsely populated multicast groups with the focus on WAN.
PIM uses the IP routing table instead of maintaining its own, thereby being routing protocol
independent.
Initiating PIM multicasts on a network
Once PIM is enabled on each router, a network user can begin a video conference multicast from
the server on R1 as shown in Figure 172. When a multicast packet is received on a PIM-capable
router interface, the interface checks its IP 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 back to the source, the multicast packet is then forwarded to all neighboring PIM
routers. Otherwise, the multicast packet is discarded and a prune message is sent back upstream.
In Figure 172, 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 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).