26-1
v1.0, June 2010
Chapter
26
PIM
In this chapter, the following examples are provided:
•
“PIM-DM Configuration”
•
“PIM-SM Configuration” on page
26-27
Multicast protocols are used to deliver multicast packets from one source to multi-receivers. They facilitate
better bandwidth utilization, and use less host and router processing, making them ideal for usage in
application such as video/audio conferencing, whiteboard tools, stock distribution tickers, and so on. PIM is
a widely used multicast routing protocol. Protocol Independent Multicast (PIM) is a standard multicast
routing protocol that provides scalable inter-domain multicast routing across the Internet, independent of the
mechanisms provided by any particular unicast routing protocol. PIM has two types:
•
PIM-Dense Mode (PIM-DM)
•
PIM-Sparse Mode (PIM-SM)
PIM-DM Configuration
PIM-DM protocol is a simple, protocol-independent multicast routing protocol. It uses existing Unicast
routing table and join/ prune/graft mechanism to build a tree. PIM-DM creates source-based shortest-path
distribution trees making use of Reverse Path Forwarding (RPF). PIM-DM cannot be used to build a shared
distribution tree, as PIM-SM can. PIM-DM assumes that when a sender starts sending data, all downstream
routers and hosts want to receive a multicast datagram. PIM-DM initially floods multicast traffic throughout
the network. Routers that do not have any downstream neighbors prune back the unwanted traffic. Apart
from the prune messages, PIM-DM makes use of two more messages: graft and assert. Graft messages are
used whenever a new host wants to join the group. Assert messages are used to shut off duplicate flows onto
the same multi-access network. To minimize the repeated flooding of datagrams and subsequent pruning
associated with a particular (S,G) pair, PIM-DM uses a State Refresh message. This message is sent by the
router(s) directly connected to the source and is propagated throughout the network. When received by a
router on its RPF interface, the State Refresh message causes an existing prune state to be refreshed. State
Refresh messages are generated periodically by the router directly attached to the source. There are two
Note:
The PIM protocol can be configured to operate on IPv4 and IPv6 networks. Separate
configuration CLI commands are provided for IPv4 and IPv6 operation; however, most
configuration options are common to both protocols. Therefore, this section describes only
IPv4 configuration, and IPv6 configuration is similar to IPv4.
