Protocol Independent Multicast (PIM) is a multicast routing protocol that
makes packet-forwarding decisions independent of standard or unicast IP routing
protocols such as EIGRP. At the same time, PIM can leverage unicast routing
tables to perform multicast forwarding functions. For example, PIM uses the
unicast routing table to perform the RPF check function. Unlike unicast routing
protocols, PIM does not send and receive routing updates between routers.
PIM has three forwarding modes:
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PIM Dense Mode (PIM-DM) – This mode uses a push model to flood
multicast traffic to every corner of the network. Routers located throughout
the tree can then prune the flow if they receive no requests for the particular
multicast group flow. This method would be efficient in certain deployments in
which there are typically active receivers on every subnet in the network.
-
PIM Sparse Mode (PIM-SM) – This mode uses a pull model to deliver
multicast traffic. Only network segments with active receivers explicitly
requesting the flow will receive the traffic for a multicast group. PIM-SM
distributes information about active sources by forwarding data packets on the
shared tree. Because PIM-SM uses shared trees, the source forwards its flow to
a rendezvous point (RP), which then sends it to any requesting routers.
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PIM Sparse-dense Mode – This mode allows individual groups to be run
in either sparse or dense mode depending on whether RP information is available
for that group. If the router gleans RP information for a particular group, it
will be treated as sparse mode; otherwise that group will be treated as dense
mode.
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NOTE:
Cisco strongly recommends sparse-dense mode.
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PIM Version 1 and 2
PIM version 1 was Cisco proprietary.
In addition to being an IEEE standard, Version 2 includes the following
improvements:
- A single, active rendezvous point (RP) exists per multicast group, with
multiple backup RPs. This compares to multiple active RPs for the same group in
PIM Version 1.
- A bootstrap router (BSR) provides a fault-tolerant, automated RP discovery
and distribution mechanism. Thus, routers dynamically learn the group-to-RP
mappings.
- Sparse mode and dense mode are properties of a group, as opposed to an
interface. PIM Join and Prune messages have more flexible encodings for
multiple address families.
- Register messages to an RP indicate whether they were sent by a border
router or by a designated router.
- PIM packets are no longer inside IGMP packets; they are stand-alone
packets.