H3C Technologies H3C S7500E Series Switches - Manual (Page 87)

4-3
The above-mentioned “packet source” can mean different things in different situations:
z For a packet traveling 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.
z For a packet traveling along the rendezvous point tree (RPT) from the RP to the receivers,
the “packet source” for RPF check is the RP.
z For a bootstrap message from the bootstrap router (BSR), the “packet source” for RPF
check is the BSR.
For details about the concepts of SPT, RPT and BSR, refer to PIM Configuration in the IP
Multicast Configuration Guide.
Implementation of RPF check 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. Upon 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
is subject to 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.
z If the interface on which the packet actually arrived is the RPF interface, the RPF check
succeeds and the router forwards the packet to all the outgoing interfaces.
z If the interface on which the packet actually arrived 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 on which the packet actually
arrived is the incoming interface, the router forwards the packet to all the outgoing
interfaces.
3) If the corresponding (S, G) entry exists, but the interface on which the packet actually
arrived is not the incoming interface in the multicast forwarding table, the multicast packet
is subject to an RPF check.
z 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 is to be discarded.
z 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 in the RPF interface, the router forwards the packet to all the outgoing
interfaces; otherwise it discards the packet.
Assume that unicast routes are available in the network, MBGP is not configured, and no
multicast static routes have been configured on Router C, as shown in Figure 4-1 . Multicast
packets travel along the SPT from the multicast source to the receivers. The multicast