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24-11
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 24 Understanding and Configuring IP Multicast
Overview of IP Multicast
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. To display the information in the hardware
tables, use the show platform hardware command.
The MFIB table contains a set of IP multicast routes. There are several types of IP multicast routes,
including (S,G) and (*,G) routes. 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:
• Internal Copy (IC) flag—set on a route when a process on the router needs to receive a copy of all
packets matching the specified route
• Signalling (S) flag—set 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 Signalling (S) flag,
except that the C flag indicates that only packets sent by directly connected hosts to the route should
be signalled to a protocol process
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 treated, and
they also indicate whether packets matching the route should be forwarded onto VLAN 1. The
per-interface flags supported in the MFIB include the following:
• Accepting (A)—set 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.
• Signalling (S)—set on an interface when some multicast routing protocol process in IOS needs to
be notified of packets arriving on that interface.
• Not platform fast-switched (NP)—used in conjunction with the Forwarding (F) flag. A Forwarding
interface is also marked as not platform fast-switched whenever that output interface cannot be fast
switched by the platform. The NP flag is typically used when the Forwarding interface cannot be
routed in hardware and requires software forwarding. For example, Catalyst 4500 series switch
tunnel interfaces are not hardware switched, so they are marked with the NP flag. If there are any
NP interfaces associated with a route, then for every packet arriving on an Accepting interface, one
copy of that packet is sent to the software forwarding path for software replication to those interfaces
that were not switched in hardware.