H3C Technologies H3C S12500 Series Switches - Manual (Page 76)

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{
The router automatically chooses an optimal static multicast route by searching its static
multicast routing table, and using the IP address of the packet source as the source address. The
corresponding routing entry explicitly defines the RPF interface and the RPF neighbor.
2. The router selects one of these optimal routes as the RPF route. The selection process is as follows:
{
If configured to use the longest prefix match principle, the router selects the longest prefix match
route from the three. If these three routes have the same mask, the router selects the route with
the highest priority. If the three routes have the same priority, the router selects a route as the RPF
route according to the sequence of static multicast route, MBGP route, and unicast route.
{
If not configured to use the longest prefix match principle, the router selects the route with the
highest priority. If the three routes have the same priority, the router selects a route as the RPF
route according to the sequence of static multicast route, MBGP route, and unicast route.
The "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 rendezvous
point (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
PIM."
RPF check implementation in multicast
Implementing an RPF check on each received multicast data packet would bring a big burden to the
router. The use of a multicast forwarding table is the solution to this issue. When creating a multicast
routing entry and a multicast forwarding entry for a multicast packet, the router sets the RPF interface of
the packet as the incoming interface of the (S, G) entry. After receiving an (S, G) multicast packet, the
router first searches its multicast forwarding table:
1. If the corresponding (S, G) entry does not exist in the multicast forwarding table, the packet
undergoes an RPF check. The router creates a multicast routing entry based on the relevant routing
information and adds the entry into the multicast forwarding table, with the RPF interface as the
incoming interface.
{
If the interface that received the packet is the RPF interface, the RPF check succeeds and the
router forwards the packet out of all the outgoing interfaces.
{
If the interface that received the packet is not the RPF interface, the RPF check fails and the router
discards the packet.
2. If the corresponding (S, G) entry exists, and the interface that received the packet is the incoming
interface, the router forwards the packet out of all the outgoing interfaces.
3. If the corresponding (S, G) entry exists, but the interface that received the packet is not the
incoming interface in the multicast forwarding table, the multicast packet undergoes an RPF check.
{
If the RPF interface is the incoming interface of the (S, G) entry, this means the (S, G) entry is
correct but the packet arrived from a wrong path. The packet will be discarded.
{
If the RPF interface is not the incoming interface, this means the (S, G) entry has expired, and
router replaces the incoming interface with the RPF interface. If the interface on which the
packet arrived is the RPF interface, the router forwards the packet out of all the outgoing
interfaces. Otherwise it discards the packet.