Pim-Source Specific Multicast - Cisco CRS Series Configuration Manual

Cisco ios xr multicast configuration guide for the cisco crs router, release 5.2.x
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Protocol Independent Multicast

PIM-Source Specific Multicast

In many multicast deployments where the source is known, protocol-independent multicast-source-specific
multicast (PIM-SSM) mapping is the obvious multicast routing protocol choice to use because of its simplicity.
Typical multicast deployments that benefit from PIM-SSM consist of entertainment-type solutions like the
ETTH space, or financial deployments that completely rely on static forwarding.
PIM-SSM is derived from PIM-SM. However, whereas PIM-SM allows for the data transmission of all sources
sending to a particular group in response to PIM join messages, the SSM feature forwards traffic to receivers
only from those sources that the receivers have explicitly joined. Because PIM joins and prunes are sent
directly towards the source sending traffic, an RP and shared trees are unnecessary and are disallowed. SSM
is used to optimize bandwidth utilization and deny unwanted Internet broadcast traffic. The source is provided
by interested receivers through IGMPv3 membership reports.
In SSM, delivery of datagrams is based on (S,G) channels. Traffic for one (S,G) channel consists of datagrams
with an IP unicast source address S and the multicast group address G as the IP destination address. Systems
receive traffic by becoming members of the (S,G) channel. Signaling is not required, but receivers must
subscribe or unsubscribe to (S,G) channels to receive or not receive traffic from specific sources. Channel
subscription signaling uses IGMP to include mode membership reports, which are supported only in Version
3 of IGMP (IGMPv3).
To run SSM with IGMPv3, SSM must be supported on the multicast router, the host where the application is
running, and the application itself. Cisco IOS XR Software allows SSM configuration for an arbitrary subset
of the IP multicast address range 224.0.0.0 through 239.255.255.255. When an SSM range is defined, existing
IP multicast receiver applications do not receive any traffic when they try to use addresses in the SSM range,
unless the application is modified to use explicit (S,G) channel subscription.
PIM-Bidirectional Mode
PIM BIDIR is a variant of the Protocol Independent Multicast (PIM) suite of routing protocols for IP multicast.
In PIM, packet traffic for a multicast group is routed according to the rules of the mode configured for that
multicast group. In bidirectional mode, traffic is only routed along a bidirectional shared tree that is rooted at
the rendezvous point (RP) for the group. In PIM-BIDIR, the IP address of the RP acts as the key to having
all routers establish a loop-free spanning tree topology rooted in that IP address. This IP address does not
need to be a router, but can be any unassigned IP address on a network that is reachable throughout the PIM
domain. Using this technique is the preferred configuration for establishing a redundant RP configuration for
PIM-BIDIR.
In Cisco IOS XR Release 4.2.1, Anycast RP is not supported on PIM Bidirectional mode.
Note
PIM-BIDIR is designed to be used for many-to-many applications within individual PIM domains. Multicast
groups in bidirectional mode can scale to an arbitrary number of sources without incurring overhead due to
the number of sources. PIM-BIDIR is derived from the mechanisms of PIM-sparse mode (PIM-SM) and
shares many SPT operations. PIM-BIDIR also has unconditional forwarding of source traffic toward the RP
upstream on the shared tree, but no registering process for sources as in PIM-SM. These modifications are
necessary and sufficient to allow forwarding of traffic in all routers solely based on the (*, G) multicast routing
entries. This feature eliminates any source-specific state and allows scaling capability to an arbitrary number
of sources.
The traditional PIM protocols (dense-mode and sparse-mode) provided two models for forwarding multicast
packets, source trees and shared trees. Source trees are rooted at the source of the traffic while shared trees
Cisco IOS XR Multicast Configuration Guide for the Cisco CRS Router, Release 5.2.x
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Implementing Multicast Routing on Cisco IOS XR Software

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