H3C Technologies H3C S12500 Series Switches - Manual (Page 75)

59
Configuring multicast routing and forwarding
Overview
In multicast implementations, multicast routing and forwarding are implemented by routing and
forwarding tables:
• Each multicast routing protocol has its own multicast routing table, such as the PIM routing table.
• The multicast routing information of different multicast routing protocols forms a general multicast
routing table.
• The multicast forwarding table helps guide the forwarding of multicast packets.
A multicast routing table consists of a set of (S, G) entries, each indicating the routing information for
delivering multicast data from a multicast source to a multicast group. If a router supports multiple
multicast protocols, its multicast routing table includes routes generated by multiple protocols. The router
chooses the optimal route from the multicast routing table based on the configured multicast routing and
forwarding policy and adds the route entry into its multicast forwarding table.
The term "router" in this document refers to both routers and Layer 3 switches.
RPF check mechanism
A multicast routing protocol relies on the existing unicast routes, MBGP routes, or static multicast routes
in creating multicast routing entries. When creating multicast routing table entries, a multicast routing
protocol uses the reverse path forwarding (RPF) check mechanism to ensure multicast data delivery along
the correct paths. In addition, the RPF check mechanism also helps avoid data loops.
The basis for an RPF check is as follows:
• Unicast routing table—Contains the shortest path to each destination subnet.
• MBGP routing table—Contains multicast routing information.
• Static multicast routing table—Contains the RPF routing information defined by the user through
static configuration.
RPF check process
When performing an RPF check, a router searches its unicast routing table, MBGP routing table, and
static multicast routing table at the same time. The process is as follows:
1. The router chooses an optimal route from the unicast routing table, MBGP routing table, and static
multicast routing table:
{
The router automatically chooses an optimal unicast route by searching its unicast routing table,
and using the IP address of the packet source as the destination address. The outgoing interface
in the corresponding routing entry is the RPF interface and the next hop is the RPF neighbor. The
router considers the path along which the packet from the RPF neighbor arrived on the RPF
interface to be the shortest path that leads back to the source.
{
The router automatically chooses an optimal MBGP route by searching its MBGP routing table,
and using the IP address of the packet source as the destination address. The outgoing interface
in the corresponding routing entry is the RPF interface and the next hop is the RPF neighbor.