3Com S7906E - Switch, Configuration Manual

Page: 1240 / 2621

1-2
RPF Check Mechanism
An IPv6 multicast routing protocol relies on the existing IPv6 unicast routing information or IPv6 MBGP
routes in creating IPv6 multicast routing entries. When creating IPv6 multicast routing table entries, an
IPv6 multicast routing protocol uses the reverse path forwarding (RPF) check mechanism to ensure
IPv6 multicast data delivery along the correct path. In addition, the RPF check mechanism also helps
avoid data loops caused by various reasons.
RPF Check process
The basis for an RPF check is an IPv6 unicast route or an IPv6 MBGP route.
An IPv6 unicast routing table contains the shortest path to each destination subnet;
An IPv6 MBGP routing table contains IPv6 multicast routing information.
When performing an RPF check, a router searches its IPv6 unicast routing table and IPv6 MBGP
routing table at the same time. The specific process is as follows:
The router first chooses an optimal route from the IPv6 unicast routing table and IPv6 MBGP routing
table respectively:
The router searches its IPv6 unicast routing table using the IPv6 address of the “packet source” as the
destination address and automatically selects the optimal route as the RPF route. 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 IPv6 multicast 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 IPv6 MBGP route by searching its MBGP routing table,
using the IPv6 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.
Then, the router selects one from these two optimal routes as the RPF route. The selection process is
as follows:
If configured to use the longest match principle, the router selects the longest match route from the two;
if these two routes have the same prefix length, the router selects the route with a higher priority; if these
two routes have the same priority, the router selects the IPv6 MBGP route as the RPF route.
If not configured to use the longest match principle, the router selects the route with a higher priority; if
these two routes have the same priority, the router selects the IPv6 MBGP route as the RPF route.