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24-5
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 24 Understanding and Configuring IP Multicast
Overview of IP Multicast
Figure 24-2 Logical View of Layer 2 and Layer 3 Forwarding in Hardware
This section contains the following subsections:
• CEF, MFIB, and Layer 2 Forwarding, page 24-5
• IP Multicast Tables, page 24-7
• Hardware and Software Forwarding, page 24-8
• Non-Reverse Path Forwarding Traffic, page 24-9
• Multicast Fast Drop, page 24-10
• Multicast Forwarding Information Base, page 24-11
• S/M, 224/4, page 24-12
CEF, MFIB, and Layer 2 Forwarding
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