Multicast Forwarding Information Base - Cisco Catalyst 4500 Series Software Configuration Manual

About IP Multicast
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.

Multicast Forwarding Information Base

The Multicast Forwarding Information Base (MFIB) subsystem supports IP multicast routing in the
Integrated Switching Engine hardware on the Catalyst 4500 series switch. The MFIB logically resides
between the IP multicast routing protocols in the CPU subsystem software (PIM, IGMP, MSDP, MBGP,
and DVMRP) and the platform-specific code that manages IP multicast routing in hardware. The MFIB
translates the routing table information created by the multicast routing protocols into a simplified
format that can be efficiently processed and used for forwarding by the Integrated Switching Engine
hardware.
To display the information in the multicast routing table, use the show ip mroute command. To display
the MFIB table information, use the show ip mfib command.
The MFIB table contains a set of IP multicast routes. IP multicast routes include (S,G) and (*,G). Each
route in the MFIB table can have one or more optional flags associated with it. The route flags indicate
how a packet that matches a route should be forwarded. For example, the Internal Copy (IC) flag on an
MFIB route indicates that a process on the switch needs to receive a copy of the packet. The following
flags can be associated with MFIB routes:
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A route can also have a set of optional flags associated with one or more interfaces. For example, an
(S,G) route with the flags on VLAN 1 indicates how packets arriving on VLAN 1 should be handled, and
whether packets matching the route should be forwarded onto VLAN 1. The per-interface flags
supported in the MFIB include the following:
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Internal Copy (IC) flag—Sets on a route when a process on the router needs to receive a copy of all
packets matching the specified route.
Signalling (S) flag—Sets on a route when a process needs to be notified when a packet matching the
route is received; the expected behavior is that the protocol code updates the MFIB state in response
to receiving a packet on a signalling interface.
Connected (C) flag—–When set on an MFIB route, has the same meaning as the Signaling (S) flag,
except that the C flag indicates that only packets sent by directly connected hosts to the route should
be signaled to a protocol process.
Accepting (A)—Sets on the interface that is known in multicast routing as the RPF interface. A
packet that arrives on an interface that is marked as Accepting (A) is forwarded to all Forwarding
(F) interfaces.
Forwarding (F)—Used in conjunction with the Accepting (A) flag as described above. The set of
Forwarding interfaces that form what is often referred to as the multicast "olist" or output interface
list.
Signaling (S)—Sets on an interface when some multicast routing protocol process in Cisco IOS
needs to be notified of packets arriving on that interface.
Chapter 38 Configuring IP Multicast