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Software Configuration Guide—Release 15.0(2)SG
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Chapter 33 Configuring IP Multicast About IP Multicast
Figure 33-6 Redundant Multicast Router Configuration in a Stub Network
In this kind of topology, only Router A, the PIM designated router (PIM DR), forwards data to the
common VLAN. Router B receives the forwarded multicast traffic, but must drop this traffic because it
has arrived on the wrong interface and fails the RPF check. Traffic that fails the RPF check is called
non-RPF traffic.
Multicast Fast Drop
In IP multicast protocols, such as PIM-SM and PIM-DM, every (S,G) or (*,G) route has an incoming
interface associated with it. This interface is referred to as the reverse path forwarding interface. In some
cases, when a packet arrives on an interface other than the expected RPF interface, the packet must be
forwarded to the CPU subsystem software to allow PIM to perform special protocol processing on the
packet. One example of this special protocol processing that PIM performs is the PIM Assert protocol.
By default, the integrated switching engine hardware sends all packets that arrive on a non-RPF interface
to the CPU subsystem software. However, processing in software is not necessary in many cases, because
these non-RPF packets are often not needed by the multicast routing protocols. The problem is that if no
action is taken, the non-RPF packets that are sent to the software can overwhelm the CPU.
To prevent this situation from happening, the CPU subsystem software loads fast-drop entries in the
hardware when it receives an RPF failed packet that is not needed by the PIM protocols running on the
switch. A fast-drop entry is keyed by (S,G, incoming interface). Any packet matching a fast-drop entry
is bridged in the ingress VLAN, but is not sent to the software, so the CPU subsystem software is not
overloaded by processing these RPF failures unnecessarily.
Protocol events, such as a link going down or a change in the unicast routing table, can impact the set of
packets that can safely be fast dropped. A packet that was correctly fast dropped before might, after a
topology change, need to be forwarded to the CPU subsystem software so that PIM can process it. The
CPU subsystem software handles flushing fast-drop entries in response to protocol events so that the
PIM code in IOS can process all the necessary RPF failures.
The use of fast-drop entries in the hardware is critical in some common topologies because you may have
persistent RPF failures. Without the fast-drop entries, the CPU is exhausted by RPF failed packets that
it did not need to process.
Router A Router B
Network A
Network B
Multicast Traffic
Non-RPF Traffic
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