Cisco ASR 9000 Series Software Manual page 9

Implementing Layer-3 Multicast Routing on Cisco IOS XR Software
If the multicast subsequent address family identifier (SAFI) is configured for Border Gateway Protocol (BGP),
or if multicast intact is configured, a separate multicast unicast RIB is created and populated with the BGP
multicast SAFI routes, the intact information, and any IGP information in the unicast RIB. Otherwise, PIM
gets information directly from the unicast SAFI RIB. Both multicast unicast and unicast databases are outside
of the scope of PIM.
The Cisco IOS XR implementation of PIM is based on RFC 4601 Protocol Independent Multicast - Sparse
Mode (PIM-SM): Protocol Specification. For more information, see RFC 4601 and the Protocol Independent
Multicast (PIM): Motivation and Architecture Internet Engineering Task Force (IETF) Internet draft.
Cisco IOS XR Software supports PIM-SM, PIM-SSM, and PIM Version 2 only. PIM Version 1 hello
Note
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
Cisco ASR 9000 Series Aggregation Services Router Multicast Configuration Guide, Release 5.1.x
Protocol Independent Multicast
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