Specifications

70 Implementing IBM System Networking 10Gb Ethernet Switches
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
becomes duplicated only 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 by 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 provides 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
segment, one router is elected as the Designated Router (DR) for handling multicasts for the
segment. DRs forward information to a similar device, the Rendezvous Point (RP), which
holds the root tree for the particular multicast group.
Receiver join requests and 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.
Supported PIM modes and features
For each interface attached to a PIM network component, PIM can be configured to operate
either in PIM Sparse Mode (PIM-SM) or PIM Dense Mode (PIM-DM).
򐂰 PIM-SM is used in networks where multicast senders and receivers comprise a relatively
small (sparse) portion of the overall network. PIM-SM uses a more complex process than
PIM-DM for collecting and optimizing multicast routes, but minimizes impact on other IP
services and is more commonly used.
򐂰 PIM-DM is used where multicast devices are a relatively large (dense) portion of the
network, with frequent (or constant) multicast traffic. PIM-DM requires less configuration
on the switch than PIM-SM, but uses broadcasts that can consume more bandwidth in
establishing and optimizing routes.
PIM Dense Mode
PIM Dense Mode, which is not commonly used today, is intended for densly populated
networks with many receivers. The PIM Dense Mode uses
Flood-Prune behavior, which
means traffic is by default flooded to each network segment. It is up to the router on the
segment to send a prune message to the source, which signals that there are no receivers
interested in the multicast traffic flooded.
For more information about PIM Dense Mode, see RFC 3973, found at:
http://www.ietf.org/rfc/rfc3973.txt