Pim-Sparse Mode - Cisco XR12000 Series Configuration Manual

Implementing Multicast Routing on Cisco IOS XR Software
Cisco IOS XR Software supports PIM-SM, PIM-SSM, PIM Bidir, and PIM Version 2 only. PIM Version 1
Note
hello messages that arrive from neighbors are rejected.

PIM-Sparse Mode

Typically, PIM in sparse mode (PIM-SM) operation is used in a multicast network when relatively few routers
are involved in each multicast. Routers do not forward multicast packets for a group, unless there is an explicit
request for traffic. Requests are accomplished using PIM join messages, which are sent hop by hop toward
the root node of the tree. The root node of a tree in PIM-SM is the rendezvous point (RP) in the case of a
shared tree or the first-hop router that is directly connected to the multicast source in the case of a shortest
path tree (SPT). The RP keeps track of multicast groups, and the sources that send multicast packets are
registered with the RP by the first-hop router of the source.
As a PIM join travels up the tree, routers along the path set up the multicast forwarding state so that the
requested multicast traffic is forwarded back down the tree. When multicast traffic is no longer needed, a
router sends a PIM prune message up the tree toward the root node to prune (or remove) the unnecessary
traffic. As this PIM prune travels hop by hop up the tree, each router updates its forwarding state appropriately.
Ultimately, the forwarding state associated with a multicast group or source is removed. Additionally, if prunes
are not explicitly sent, the PIM state will timeout and be removed in the absence of any further join messages.
PIM-SM is the best choice for multicast networks that have potential members at the end of WAN links.
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.
OL-30405-02
Cisco IOS XR Multicast Configuration Guide for the Cisco XR12000 Series Router, Release 5.1.x
Protocol Independent Multicast
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