Pim Overview - Lenovo RackSwitch G8264 Application Manual

PIM Overview

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G8264 Application Guide for ENOS 8.4
PIM is designed for efficiently routing multicast traffic across one or more IPv4
domains. This has benefits for application such as IP television, collaboration,
education, and software delivery, where a single source must deliver content (a
multicast) to a group of receivers that span both wide‐area and inter‐domain
networks.
Instead of sending a separate copy of content to each receiver, a multicast derives
efficiency by sending only a single copy of content toward its intended receivers.
This single copy only becomes duplicated when it reaches the target domain that
includes multiple receivers, or when it reaches a necessary bifurcation point
leading to different receiver domains.
PIM is used by multicast source stations, client receivers, and intermediary routers
and switches, to build and maintain efficient multicast routing trees. PIM is
protocol independent; It collects routing information using the existing unicast
routing functions underlying the IPv4 network, but does not rely on any particular
unicast protocol. For PIM to function, a Layer 3 routing protocol (such as BGP,
OSPF, RIP, or static routes) must first be configured on the switch.
PIM‐SM is a reverse‐path routing mechanism. Client receiver stations advertise
their willingness to join a multicast group. The local routing and switching devices
collect multicast routing information and forward the request toward the station
that will provide the multicast content. When the join requests reach the sending
station, the multicast data is sent toward the receivers, flowing in the opposite
direction of the original join requests.
Some routing and switching devices perform special PIM‐SM functions. Within
each receiver domain, one router is elected as the Designated Router (DR) for
handling multicasts for the domain. DRs forward information to a similar device,
the Rendezvous Point (RP), which holds the root tree for the particular multicast
group.
Receiver join requests as well as sender multicast content initially converge at the
RP, which generates and distributes multicast routing data for the DRs along the
delivery path. As the multicast content flows, DRs use the routing tree information
obtained from the RP to optimize the paths both to and from send and receive
stations, bypassing the RP for the remainder of content transactions if a more
efficient route is available.
DRs continue to share routing information with the RP, modifying the multicast
routing tree when new receivers join, or pruning the tree when all the receivers in
any particular domain are no longer part of the multicast group.