RPF check mechanism

A multicast routing protocol uses reverse path forwarding (RPF) check to ensure the multicast data delivery along the correct path and to avoid data loops.

A multicast device performs the RPF check on a multicast packet as follows:

1. Chooses an optimal route back to the packet source separately from the unicast, MBGP, and static multicast routing tables.

   The term "packet source" means different things in different situations:

   • For a packet that travels along the SPT, the packet source is the multicast source.

   • For a packet that travels along the RPT, the packet source is the RP.

   • For a bootstrap message originated from the BSR, the packet source is the BSR.

   For more information about the concepts of SPT, RPT, source-side RPT, RP, and BSR, see "Configuring PIM."

2. Selects one of the three optimal routes as the RPF route as follows:

   • If the device uses the longest prefix match principle, the route with the highest subnet mask becomes the RPF route. If the routes have the same mask, the route with the highest route preference becomes the RPF route. If the routes have the same route preference, the unicast route becomes the RPF route.

     For more information about the route preference, see Layer 3—IP Routing Configuration Guide.

   • If the device does not use the longest prefix match principle, the route with the highest route preference becomes the RPF route. If the routes have the same preference, the unicast route becomes the RPF route.

   The RPF route contains the RPF interface and RPF neighbor information.

   • If the RPF route is a unicast route or MBGP route, the outgoing interface is the RPF interface and the next hop is the RPF neighbor.

   • If the RPF route is a static multicast route, the RPF interface and RPF neighbor are specified in the route.

3. Determines whether the packet arrived at the RPF interface.

   • If the packet arrived at the RPF interface, the RPF check succeeds and the packet is forwarded.

   • If the packet arrived at the non-RPF interface, the RPF check fails and the packet is discarded.

Implementing an RPF check on each received multicast packet brings a big burden to the device. The use of a multicast forwarding table is the solution to this issue. When the device creates a multicast forwarding entry for an (S, G) packet, it sets the RPF interface of the packet as the incoming interface of the (S, G) entry. After the device receives another (S, G) packet, it looks up the multicast forwarding table for a matching (S, G) entry.

• If no match is found, the device first determines the RPF route back to the packet source and the RPF interface. Then, it creates a forwarding entry with the RPF interface as the incoming interface and makes the following judgments:

  • If the receiving interface is the RPF interface, the RPF check succeeds and the device forwards the packet out of all the outgoing interfaces.

  • If the receiving interface is not the RPF interface, the RPF check fails and the device discards the packet.

• If a match is found and the matching forwarding entry contains the receiving interface, the device forwards the packet out of all the outgoing interfaces.

• If a match is found but the matching forwarding entry does not contain the receiving interface, the device determines the RPF route back to the packet source. Then, the device performs one of the following actions:

  • If the RPF interface is the incoming interface, it means that the forwarding entry is correct but the packet traveled along a wrong path. The packet fails the RPF check, and the device discards the packet.

  • If the RPF interface is not the incoming interface, it means that the forwarding entry has expired. The device replaces the incoming interface with the RPF interface and matches the receiving interface against the RPF interface. If the receiving interface is the RPF interface, the device forwards the packet out of all outgoing interfaces. Otherwise, it discards the packet.

As shown in Figure 27, assume that unicast routes are available on the network, MBGP is not configured, and no static multicast routes have been configured on Device C. Multicast packets travel along the SPT from the multicast source to the receivers. The multicast forwarding table on Device C contains the (S, G) entry, with Port A as the incoming interface.

• If a multicast packet arrives at Device C on Port A, the receiving interface is the incoming interface of the (S, G) entry. Device C forwards the packet out of all outgoing interfaces.

• If a multicast packet arrives at Device C on Port B, the receiving interface is not the incoming interface of the (S, G) entry. Device C searches its unicast routing table and finds that the outgoing interface to the source (the RPF interface) is Port A. In this case, the (S, G) entry is correct, but the packet traveled along a wrong path. The packet fails the RPF check and Device C discards the packet.