HP VPN Firewall Appliances Appendix Protocol Reference

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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 the PIM routing table.
• General multicast routing table—The multicast routing information of different multicast routing
protocols forms a general multicast routing table.
• Multicast forwarding 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 entry contains 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 contains all 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 routing-capable firewalls.
RPF check mechanism
A multicast routing protocol relies on the existing unicast 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
A multicast routing protocol uses the following tables to perform the RPF check:
• Unicast routing table—Contains unicast routing information.
• Static multicast routing table—Contains RPF routes that are manually configured.
When a firewall performs an RPF check, it searches its unicast routing table and static multicast routing
table. The specific process of RPF check is as follows:
1. The router separately chooses an optimal route from the unicast routing table and the static
multicast routing table:
{ The router searches its unicast routing table by using the IP address of the packet source as the
destination address and automatically chooses an optimal unicast route. 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.