The implementation of IP multicast on the Catalyst 4500 series switch is an extension of centralized
Cisco Express Forwarding (CEF). CEF extracts information from the unicast routing table, which is
created by unicast routing protocols, such as BGP, OSPF, and EIGR and loads it into the hardware
Forwarding Information Base (FIB). With the unicast routes in the FIB, when a route is changed in the
upper-layer routing table, only one route needs to be changed in the hardware routing state. To forward
unicast packets in hardware, the Integrated Switching Engine looks up source and destination routes in
ternary content addressable memory (TCAM), takes the adjacency index from the hardware FIB, and
gets the Layer 2 rewrite information and next-hop address from the hardware adjacency table.

The new Multicast Forwarding Information Base (MFIB) subsystem is the multicast analog of the
unicast CEF. The MFIB subsystem extracts the multicast routes that PIM and IGMP create and refines
them into a protocol-independent format for forwarding in hardware. The MFIB subsystem removes the
protocol-specific information and leaves only the essential forwarding information. Each entry in the
MFIB table consists of an (S,G) or (*,G) route, an input RPF VLAN, and a list of Layer 3 output
interfaces. The MFIB subsystem, together with platform-dependent management software, loads this
multicast routing information into the hardware FIB and hardware multicast expansion table (MET).

Integrated Switching Engine (ASIC)

L3 physical
interface

Logical Router

L3 logical
interfaces

L2 switchports

68402

VLAN1

Gig 1/1

VLAN2