3Com MSR 50 Series, Руководство по настройке

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1210 C
HAPTER
72: IP
V
6 M
ULTICAST
R
OUTING AND
F
ORWARDING
C
ONFIGURATION
Implementation of the RPF mechanism
Upon receiving an IPv6 multicast packet sent from a multicast source S to an IPv6
multicast group G, the router first searches its IPv6 multicast forwarding table:
1
If the corresponding (S, G) entry exists, and the interface on which the packet
actually arrived is the incoming interface in the IPv6 multicast forwarding table,
the router forwards the packet to all the outgoing interfaces.
2
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 packet is subject to an RPF check.
■
If the result of the RPF check shows that the RPF interface is the incoming
interface of the existing (S, G) entry, this means that the (S, G) entry is correct
but the packet arrived from a wrong path and is to be discarded.
■
If the result of the RPF check shows that the RPF interface is not the incoming
interface of the existing (S, G) entry, this means that the (S, G) entry is no
longer valid. The router replaces the incoming interface of the (S, G) entry with
the interface on which the packet actually arrived and forwards the packet to
all the outgoing interfaces.
3
If no corresponding (S, G) entry exists in the multicast forwarding table, the packet
is also subject to an RPF check. The router creates an (S, G) entry based on the
relevant routing information and using the RPF interface as the incoming interface,
and installs the entry into the IPv6 multicast forwarding table.
■
If the interface on which the packet actually arrived is the RPF interface, the RPF
check is successful 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.
RPF Check
The basis for an RPF check is an IPv6 unicast route. The IPv6 unicast routing table
contains the shortest path to each destination subnet. A multicast routing protocol
does not independently maintain any type of unicast routes; instead, it relies on
the existing unicast routing information in creating multicast routing entries.
■
When performing an RPF check, the router searches its IPv6 unicast routing
table using the IP address of the “packet source” as the destination address
and automatically selects the optimal route as the RPF route. The outgoing
interface in the corresponding routing entry is the RPF interface and the next
hop is the RPF neighbor. The router considers the path along which the IPv6
multicast packet from the RPF neighbor arrived on the RPF interface to be the
shortest path that leads back to the source.
n
The above-mentioned “packet source” can mean different things in different
situations
■
For a packet traveling along the shortest path tree (SPT) from the multicast
source to the receivers or the source-based tree from the multicast source to
the rendezvous point (RP), “packet source” means the multicast source.
■
For a packet traveling along the rendezvous point tree (RPT) from the RP to the
receivers, “packet source” means the RP.