RPF check mechanism

A multicast routing protocol relies on the existing unicast routes, MBGP routes, or static multicast routes to create multicast routing entries. When creating multicast routing entries, the multicast routing protocol uses the reverse path forwarding (RPF) check to ensure the multicast data delivery along the correct paths. The RPF check also helps avoid data loops.

A multicast routing protocol uses the following tables to perform an RPF check:

• Unicast routing table—Contains unicast routing information.

• Static multicast routing table—Contains RPF routes that are manually configured.

A static multicast routing table is used for RPF check rather than multicast routing.

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

1. The router chooses an optimal route back to the packet source separately from the unicast routing table and the static multicast routing table.

   A "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 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. The router selects one of the two optimal routes as the RPF route as follows:

   • If the router uses the longest prefix match principle, the route with a higher subnet mask becomes the RPF route. If the routes have the same mask, the route with a higher 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 router does not use the longest prefix match principle, the route with a higher route preference becomes the RPF route. If the routes have the same preference, the unicast route becomes the RPF route.

   The RPF route determines the RPF interface and the RPF neighbor.

   • If the RPF route is a unicast route, the outgoing interface of the route 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 those specified in the route.

3. The router checks whether the packet arrived at the RPF interface. If yes, the RPF check succeeds and the packet is forwarded. If not, the RPF check fails and the packet is discarded.

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

• If no match is found, the router first determines the RPF route back to the packet source. Then, it creates a forwarding entry with the RPF interface as the incoming interface and performs one of the following actions:

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

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

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

• If a match is found but the receiving interface is not the incoming interface of the forwarding entry, the router first determines the RPF route back to the packet source. Then, the router 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 router discards the packet.

  • If the RPF interface is not the incoming interface, it means that the forwarding entry has expired. The router 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 router forwards the packet out of all outgoing interfaces. Otherwise, it discards the packet.

As shown in Figure 22, assume that unicast routes are available in the network and no static multicast routes have been configured on Switch C. Multicast packets travel along the SPT from the multicast source to the receivers. The multicast forwarding table on Switch C contains the (S, G) entry, with VLAN-interface 20 as the incoming interface.

• If a multicast packet arrives at Switch C on VLAN-interface 20, the receiving interface is the incoming interface of the (S, G) entry. Switch C forwards the packet out of all outgoing interfaces.

• If a multicast packet arrives at Switch C on VLAN-interface 10, the receiving interface is not the incoming interface of the (S, G) entry. Switch C looks up its unicast routing table and finds that the outgoing interface to the source (the RPF interface) is VLAN-interface 20. In this case, the (S, G) entry is correct, but the packet traveled along a wrong path. The packet fails the RPF check and Switch C discards the packet.