3Com S7906E - Switch, Configuration Manual

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1-3
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, RP and BSR, refer to IPv6 PIM Configuration in the IP
Multicast Volume .
Implementation of the RPF check in IPv6 multicast
Implementing an RPF check on each received IPv6 multicast data packet would bring a big burden to
the router. The use of an IPv6 multicast forwarding table is the solution to this issue. When creating an
IPv6 multicast routing entry and an IPv6 multicast forwarding entry for an IPv6 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) IPv6 multicast packet, the router first searches its IPv6 multicast forwarding table:
1) If the corresponding (S, G) entry does not exist in the IPv6 multicast forwarding table, the packet is
subject to an RPF check. The router creates an IPv6 multicast routing entry based on the relevant
routing information and installs the entry into the IPv6 multicast forwarding table, with the RPF
interface as the incoming interface.
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.
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 IPv6 multicast forwarding table, the IPv6 multicast packet is subject to
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 is to 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 in the
RPF interface, the router forwards the packet to all the 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 Figure
1-1 . The IPv6 multicast forwarding table on Router C contains the (S, G) entry, with Vlan-interface20 as
the RPF interface.