Configuring Multicast Routing And Forwarding; Overview; Rpf Check Mechanism - HP 6125G Configuration Manual

Configuring multicast routing and forwarding

Overview

In multicast implementations, the following types of tables implement multicast routing and forwarding:
Multicast routing table of a multicast routing protocol—Each multicast routing protocol has its own
•
multicast routing table, such as PIM routing table.
General multicast routing table—The multicast routing information of different multicast routing
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protocols forms a general multicast routing table.
Multicast forwarding table—The multicast forwarding table guides the forwarding of multicast
•
packets.
A multicast routing table consists of a set of (S, G) entries. Each entry indicates 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 to 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.
RPF check process
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.
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• Static multicast routing table—Contains the RPF routing information defined by the user through
static configuration.
MBGP multicast routing table and static multicast routing table are used for RPF check rather than
multicast routing.
When a router performs an RPF check, it searches its unicast routing table, MBGP routing table, and
static multicast routing table at the same time. The specific process is as follows:
The router chooses an optimal route from the unicast routing table, the MBGP routing table, and
1.
the 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.
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