Information Technology Reference
In-Depth Information
The flood-and-prune behavior is not graceful and can have a dramatic impact on network
performance. A stable network has periodic traffic oscillations because of this flood-and-
prune behavior. Also, PIM-DM does not work with MSDP and does not participate with
interdomain multicast. It is generally recommended that you deploy PIM-SM and not
PIM-DM for all environments.
Sparse Mode
The PIM protocol when operating in sparse mode (PIM-SM) uses an explicit join method-
ology with no periodic flood-and-prune. The flood-and-prune method of PIM-DM provides
a way to inform all routers in the network of available multicast groups and sources. In
PIM-SM, on the other hand, the concept of the Rendezvous Point (RP) is introduced as a
way for sources and receivers to (as the name describes) rendezvous.
The RP knows about all sources and groups in the network. When a source begins sending,
its immediate upstream router registers that (S,G) with the RP. When a receiver starts
listening to a stream, if the upstream does not have a specific (S,G) or (*,G) for that stream,
it builds a shared tree (*,G) back to the RP.
PIM-SM has a couple of advantages over PIM-DM. The flood-and-prune behavior is not
used, thereby reducing network resource requirements. The second advantage is that,
unlike PIM-DM, state is not maintained in every router for all (S,G)s, not even for those
with no downstream listeners,.
The PIM-SM configuration is very similar to PIM-DM. Therefore, IP multicast routing
must be enabled for the router with the global configuration command
ip multicast-
routing
.
Every interface must be configured to enable the PIM protocol in sparse mode with the
interface command
ip pim sparse-mode
.
In addition, every router must be configured with the RP's IP address by using the global
command
ip pim rp-address
address
.
It is very important that all routers agree on the same RP for any particular multicast group.
Unlike PIM-DM, traffic can be forwarded using the (*,G) or a more specific (S,G). This
means that for traffic forwarded using the shared tree, the RPF function is performed using
the RP address. Traffic forwarded down a source tree using an (S,G) uses the source address
for the RPF function.
PIM-SM Example
This example provides an overview of the operation of a PIM-SM network. Figure 11-12
shows the initial topology. This example focuses at a high level on how PIM-SM operates,
without delving too deeply into the protocol state details.
