Cisco 4500M Software Manual page 336

Overview of IP Multicast
Figure 25-6 Redundant Multicast Router Configuration in a Stub Network
Multicast Traffic
Non-RPF Traffic
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.
Use the ip mfib fastdrop command to enable or disable MFIB fast drops.
To prevent this 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 it is possible
to have persistent RPF failures. Without the fast-drop entries, the CPU would be exhausted by RPF failed
packets that it did not need to process.
Software Configuration Guide—Release 12.2(25)EW
25-10
Router A Router B
Network A
Network B
Chapter 25 Understanding and Configuring IP Multicast
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