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Implementing Multicast Routing on Cisco IOS XR Software
When configuring IGMP, ensure that all systems on the subnet support the same IGMP version. The router
Note
does not automatically detect Version 1 systems. Configure the router for Version 2 if your hosts do not
support Version 3.
Protocol Independent Multicast
Protocol Independent Multicast (PIM) is a routing protocol designed to send and receive multicast routing
updates. Proper operation of multicast depends on knowing the unicast paths towards a source or an RP. PIM
relies on unicast routing protocols to derive this reverse-path forwarding (RPF) information. As the name
PIM implies, it functions independently of the unicast protocols being used. PIM relies on the Routing
Information Base (RIB) for RPF information.
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.
Note
Cisco IOS XR Software supports PIM-SM, PIM-SSM, PIM Bidir, and PIM Version 2 only. PIM Version 1
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.
Cisco IOS XR Multicast Configuration Guide for the Cisco CRS Router, Release 5.2.x
Protocol Independent Multicast
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