Explains the multicast-intact feature, covering its role in enabling multicast and unicast routing coexistence, protocol support, interaction with MPLS tunnels, management of next-hops, and implementation considerations for IS-IS and OSPFv2.
Multicast-intact is a routing protocol capability that
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enables multicast routing protocols (such as PIM) to coexist with IGP shortcuts (such as MPLS tunnels) on a router
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provides a parallel set of equal-cost next-hops for use by multicast protocols, and
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ensures these next-hops are guaranteed not to contain any IGP shortcuts and are not used for unicast routing or published to the FIB.
Supported protocols
Multicast-intact feature is supported by both OSPFv2 and IS-IS.
Coexistence with MPLS TE and IP multicast
Multicast-intact feature allows MPLS Traffic Engineering (TE) and IP multicast to coexist on the same router.
Publishing multicast-intact next-hops
When multicast-intact is enabled, all IPv4 destinations learned through link-state advertisements are published with a set of equal-cost mcast-intact next-hops to the RIB.This applies even if the native next-hops have no IGP shortcuts.
Next-hop limits in IS-IS and OSPFv2
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In IS-IS, the max-paths limit is applied by counting both native and multicast-intact next-hops together.
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OSPFv2 handles multicast-intact next-hops with its own unique behavior.
Enabling multicast-intact allows PIM protocols to function correctly by providing eligible next-hops, even when the router uses MPLS tunnels as IGP shortcuts.