375
2.
The router selects one of the optimal routes as the RPF route according to the following principles:
{
If the router uses the longest prefix match principle, it selects the longest matching route as the
RPF route. If the routes have the same prefix length, the router selects the route that has a higher
priority as the RPF route. If the routes have the same priority, the router selects the IPv6 MBGP
route as the RPF route.
{
If the router does not use the longest prefix match principle, it selects the route that has a higher
priority as the RPF route. If the routes have the same priority, the router selects the IPv6 MBGP
route as the RPF route.
The term "packet source" means different things in different situations:
•
For a packet that travels along the SPT from the multicast source to the receivers or the RP, the packet
source for RPF check is the multicast source.
•
For a packet that travels along the RPT from the RP to the receivers, or along the source-side RPT from
the multicast source to the RP, the packet source for RPF check is the RP.
•
For a bootstrap message from the BSR, the packet source for RPF check is the BSR.
For more information about the concepts of SPT, RPT, source-side RPT, RP, and BSR, see "Configuring IPv6
PIM."
RPF check implementation in IPv6 multicast
Implementing an RPF check on each received IPv6 multicast packet would heavily burden the router. The
use of an IPv6 multicast forwarding table is the solution to this issue. When the router creates an IPv6
multicast routing entry and an IPv6 multicast forwarding entry for an IPv6 multicast packet, it sets the RPF
interface of the packet as the incoming interface of the forwarding entry. After the router receives an IPv6
multicast packet, it searches its IPv6 multicast forwarding table:
•
If no forwarding entry matches the packet, the packet undergoes an RPF check. The router creates
an IPv6 multicast routing entry with the RPF interface as the incoming interface and installs the entry
into the IPv6 multicast forwarding table.
{
If the interface that received the packet is the RPF interface, the RPF check succeeds and the
router forwards the packet to all outgoing interfaces.
{
If the interface that received the packet is not the RPF interface, the RPF check fails and the router
discards the packet.
•
If a forwarding entry matches the packet, and the interface that received the packet is the incoming
interface of the forwarding entry, the router forwards the packet to all outgoing interfaces.
•
If a forwarding entry matches the packet, but the interface that received the packet is not the
incoming interface of the forwarding entry, the IPv6 multicast packet undergoes an RPF check.
{
If the RPF interface is the incoming interface, it indicates that the forwarding entry is correct but
the packet traveled along a wrong path. The router discards the packet.
{
If the RPF interface is not the incoming interface, it indicates that the forwarding entry has
expired, and the router replaces the incoming interface with the RPF interface. If the interface
that received the packet is the RPF interface, the router forwards the packet to all outgoing
interfaces. Otherwise, it discards the packet.
As shown in
, assume that IPv6 unicast routes are available in the network, IPv6 MBGP is not
configured, and IPv6 multicast packets travel along the SPT from the multicast source to the receivers. The
IPv6 multicast forwarding table on Router C contains the (S, G) entry, with POS 5/1 as the RPF interface.