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Configuring multicast routing and forwarding
Overview
The following tables are involved in multicast routing and forwarding:
• Multicast routing table of each multicast routing protocol, such as the PIM routing table.
• General multicast routing table that summarizes multicast routing information generated by different
multicast routing protocols. The multicast routing information from multicast sources to multicast
groups are stored in a set of (S, G) routing entries.
• Multicast forwarding table that guides multicast forwarding. The optimal routing entries in the
multicast routing table are added to the multicast forwarding table.
The term "interface" in this chapter collectively refers to Layer 3 interfaces, including VLAN interfaces
and Layer 3 Ethernet interfaces. You can set an Ethernet port as a Layer 3 interface by using the port
link-mode route command (see Layer 2
—
LAN Switching Configuration Guide).
RPF check mechanism
A multicast routing protocol creates multicast routing entries based on the existing unicast routes or static
multicast routes. During the process, the reverse path forwarding (RPF) check mechanism ensures the
multicast data delivery along the correct paths and avoids 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.
RPF check process
When performing an RPF check, the router searches its unicast routing table and static multicast routing
table at the same time using the following process:
1. The router separately chooses an optimal route from the unicast routing table and the static
multicast routing table:
{ The router looks up its unicast routing table for an optimal routing entry back to the packet
source. The outgoing interface of the route is the RPF interface and the next hop is the RPF
neighbor. The router considers the path of the packet that the RPF interface receives from the
RPF neighbor as the shortest path that leads back to the source.
{ The router looks up its static multicast routing table for an optimal routing entry back to the
packet source. The route explicitly defines the RPF interface and the RPF neighbor.
2. The router selects one of the two optimal routes as the RPF route according to the following
principles:
{ If the router uses the longest prefix match principle, the router selects the matching route as the
RPF route. If the routes have the same mask, the router selects the route that has the highest
priority as the RPF route. If the routes have the same priority, the router selects a route as the RPF
route in the order of static multicast route and unicast route.
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