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Introduction Troubleshooting PIM-SM
page 16-2 OmniSwitch Troubleshooting Guide September 2005
Introduction
Traditional multicast routing protocols like DVMRP, MOSPF or PIM-DM were implemented to provide
multicast routing in campus network. These traditional dense mode multicast protocol were intended for
use within regions where a group is widely represented or bandwidth is not an issue. However when group
members and senders to these groups are sparsely distributed across a wide area, traditional multicast rout-
ing protocol schemes do not provide an efficient way to establish distribution trees. For instance member-
ship reports or data packets are being eventually forwarded over many links where no receivers or senders
are located.
PIM-SM architecture provides a way for efficiently routing to multicast groups that may span wide area
Internets. PIM-SM, including those with WAN links, scales well to a network of any size. The explicit join
mechanism will prevent unwanted traffic from flooding the WAN links. Data multicast traffic will be
forwarded only to networks segment that have active receivers which have specifically requested the data.
PIM-SM uses a shared tree to distribute the information about active sources. Depending on the configura-
tion options the traffic can remain on the shared tree or switch over to an optimized source distribution tree
called Shortest Path Tree, SPT. The traffic starts to flow down the shared tree and then routers along the
path determine if there is a better path to the source. If a better, more direct path exists the designated
router (router closest to the receiver) will send a “join” message towards the source and then re-route the
traffic along this path.
PIM-SM uses the concept of Rendezvous Point (RP). Sources register with the RP and then data is
forwarded down the shared tree to the receivers. If the shared tree is not an optimal path between the
source and the receiver the routers will dynamically create a source tree and stop traffic from flowing
down the shared tree.
Definition of Terms
Bootstrap Router (BSR). A BSR is dynamically elected between the C-BSR (candidates BSR) within a
PIM-SM domain. Bootstrap messages are sent to discover all C-BSR and associated CBSR priority. The
BSR is the router with the highest CBSR priority. It is responsible for sending bootstrap messages, which
contains RP-Set.
Designated Router (DR). The DR is the highest IP addressed PIM-SM router on a LAN segment. It is
responsible for sending corresponding Join/Prune messages to the RP on behalf of directly connected
receivers and sources.
Rendezvous Point (RP). Each multicast group has a shared-tree via which receivers receives data from
sources. The RP is the root of this per-group shared tree, called RP tree. C-RPs (Candidates RP) are PIM-
SM routers configured to eventually become RP for some or all multicast groups address. Priority can also
be configured for a C-RP and will be used on DR when membership to a multicast group is required.
RP-Set. List of reachable C-RP sent in bootstrap messages distributed by the BSR to all PIM-SM router in
the domain. The BSR compiles the list based on C-RP advertisement. C-RPs periodically unicast C-RP-
Advertisements to the BSR for that domain The RP-Set details each C-RP with their group multicast
address availability. DRs store these bootstrap messages and use it when membership to a specific multi-
cast group is required.
RP Tree (Shared Tree). The set of paths connecting all receivers of a group to the RP.
Shortest Path Tree (SPT). The SPT is the multicast distribution tree that connects, using the shortest
path, receivers of a specific group to the source. The SPT computation is based on unicast routing but is
not depending on any particular unicast routing protocol.