363
{
The router searches its IPv6 MBGP routing table and automatically selects an optimal
MBGP route to the packet source address. The outgoing interface of the route is the RPF
interface and the next hop is the RPF neighbor.
2.
The router selects one of the optimal routes as the RPF route according to the following
principles:
{
If the router uses the longest 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 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 packet source can mean different things in different situations:
•
For a packet that travels along the shortest path tree (SPT) from the multicast source to the
receivers or the rendezvous point (RP), the packet source for RPF check is the multicast
source.
•
For a packet that travels along the rendezvous point tree (RPT) from the RP to the receivers,
the packet source for RPF check is the RP.
•
For a packet that travels 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
"
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 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 match is found, the switch determines the RPF route back to the packet source. Then, it
create a forwarding entry with the RPF interface as the incoming interface and makes the
following judgments:
{
If the receiving interface is the RPF interface, the RPF check succeeds and the router
forwards the packet to all outgoing interfaces.
{
If the receiving interface is not the RPF interface, the RPF check fails and the router
discards the packet.
•
If a forwarding entry matches the packet, and the receiving interface 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 receiving interface is not the incoming
interface of the forwarding entry, the switch determines the RPF route back to the packet source.
Then, it makes the following judgments on the packet:
{
If the RPF interface is the incoming interface, it means 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 means that the forwarding entry has
expired. The router replaces the incoming interface with the RPF interface and matches the
receiving interface against 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.
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, as shown in
