PIM sparse mode
(PIM-SM) uses a pull model to deliver multicast traffic. Only network segments
with active receivers that have explicitly requested the data will receive the
Unlike dense mode
interfaces, sparse mode interfaces are added to the multicast routing table
only when periodic Join messages are received from downstream routers, or when
a directly connected member is on the interface. When forwarding from a LAN,
sparse mode operation occurs if an RP is known for the group. If so, the
packets are encapsulated and sent toward the RP. When no RP is known, the
packets are dropped. If the multicast traffic from a specific source is
sufficient, the first hop router of the receiver may send Join messages toward
the source to build a source-based distribution tree.
information about active sources by forwarding data packets on the shared tree.
Because PIM-SM uses shared trees (at least, initially), it requires the use of
a rendezvous point (RP). The RP must be administratively configured in the
network. See the
section for more information.
In sparse mode, a
router assumes that other routers do not want to forward multicast packets for
a group, unless there is an explicit request for the traffic. When hosts join a
multicast group, the directly connected routers send PIM Join messages toward
the RP. The RP keeps track of multicast groups. Hosts that send multicast
packets are registered with the RP by the first hop router of that host. At
this point, packets are forwarded on a shared distribution tree. If the
multicast traffic from a specific source is sufficient, the first hop router of
the host may send Join messages toward the source to build a source-based
Sources register with
the RP and then data is forwarded down the shared tree to the receivers. The
edge router sends PIM (S,G) Join messages toward that source. Each router along
the reverse path compares the unicast routing metric of the RP address to the
metric of the source address. If the metric for the source address is better,
it will forward a PIM (S,G) Join message toward the source. If the metric for
the RP is the same or better, then the PIM (S,G) Join message will be sent in
the same direction as the RP. In this case, the shared tree and the source tree
would be considered congruent.
If the shared tree is
not an optimal path between the source and the receiver, the routers
dynamically create a source tree and stop traffic from flowing down the shared
tree. This behavior is the default behavior in software. Network administrators
can force traffic to stay on the shared tree by using the
infinity command. The default value of
infinity command is 0.
PIM-SM scales well to
a network of any size, including those with WAN links. The explicit join
mechanism prevents unwanted traffic from flooding the WAN links.