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Routing & Switching · Intermediate

Reverse Path Forwarding Check (RPF)

The multicast loop-prevention check that accepts a packet only if it arrives on the interface the router would use to reach the source (or RP).

Definition

RPF uses the unicast routing table (or an MRIB/static mroute). Failed RPF checks cause drops, often when unicast and multicast topologies are incongruent. Static mroutes or MBGP fix RPF failures.

Example

ip mroute 10.1.1.0 255.255.255.0 Tunnel0 fixes RPF for sources over a GRE tunnel.

Exam tip

Multicast over a GRE tunnel commonly fails RPF until a static mroute is added.

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