Multicast-Intact - Cisco CRS Series Configuration Manual

Multicast-Intact

traffic from the RP on the RPT, the router initiates a switch to a data distribution tree rooted at the source
sending traffic. This type of distribution tree is called a shortest path tree or source tree. By default, the
Cisco IOS XR Software switches to a source tree when it receives the first data packet from a source.
The following process describes the move from shared tree to source tree in more detail:
1 Receiver joins a group; leaf Router C sends a join message toward RP.
2 RP puts link to Router C in its outgoing interface list.
3 Source sends data; Router A encapsulates data in Register and sends it to RP.
4 RP forwards data down the shared tree to Router C and sends a join message toward Source. At this point,
data may arrive twice at the RP, once encapsulated and once natively.
5 When data arrives natively (unencapsulated) at RP, RP sends a register-stop message to Router A.
6 By default, receipt of the first data packet prompts Router C to send a join message toward Source.
7 When Router C receives data on (S,G), it sends a prune message for Source up the shared tree.
8 RP deletes the link to Router C from outgoing interface of (S,G). RP triggers a prune message toward
Source.
Join and prune messages are sent for sources and RPs. They are sent hop by hop and are processed by each
PIM router along the path to the source or RP. Register and register-stop messages are not sent hop by hop.
They are exchanged using direct unicast communication between the designated router that is directly connected
to a source and the RP for the group.
Tip The spt-threshold infinity command lets you configure the router so that it never switches to the shortest
path tree (SPT).
Multicast-Intact
The multicast-intact feature provides the ability to run multicast routing (PIM) when Interior Gateway Protocol
(IGP) shortcuts are configured and active on the router. Both Open Shortest Path First, version 2 (OSPFv2),
and Intermediate System-to-Intermediate System (IS-IS) support the multicast-intact feature. Multiprotocol
Label Switching Traffic Engineering (MPLS-TE) and IP multicast coexistence is supported in Cisco IOS XR
Software by using the mpls traffic-eng multicast-intact IS-IS or OSPF router command. See Cisco IOS XR
Routing Configuration Guide for the Cisco CRS Router for information on configuring multicast intact using
IS-IS and OSPF commands.
You can enable multicast-intact in the IGP when multicast routing protocols (PIM) are configured and IGP
shortcuts are configured on the router. IGP shortcuts are MPLS tunnels that are exposed to IGP. The IGPs
route the IP traffic over these tunnels to destinations that are downstream from the egress router of the tunnel
(from an SPF perspective). PIM cannot use IGP shortcuts for propagating PIM joins because reverse path
forwarding (RPF) cannot work across a unidirectional tunnel.
When you enable multicast-intact on an IGP, the IGP publishes a parallel or alternate set of equal-cost next-hops
for use by PIM. These next-hops are called mcast-intact next-hops. The mcast-intact next-hops have the
following attributes:
• They are guaranteed not to contain any IGP shortcuts.
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