Cef, Mfib, And Layer 2 Forwarding - Cisco Catalyst 4500 series Administration Manual

About IP Multicast

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 EIGRP 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 Replica Expansion Table (RET).
The Catalyst 4500 series switch performs Layer 3 routing and Layer 2 bridging at the same time. There
can be multiple Layer 2 switch ports on any VLAN interface.
Figure 37-3
routing, multicast routing, and Layer 2 bridging information to forward in hardware.
Figure 37-3
Like the CEF unicast routes, the MFIB routes are Layer 3 and must be merged with the appropriate
Layer 2 information. The following example shows an MFIB route:
(*,224.1.2.3)
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)E
37-6
more time is required for the entries to be updated in the MFIB. To reduce the traffic loss in this
scenario, you should increase the multicast route-flush timer (using the ip multicast redundancy
routeflush maxtime command) to a value exceeding the default (30 seconds).
shows a functional overview of how the Catalyst 4500 series switch combines unicast
Combining CEF, MFIB, and Layer 2 Forwarding Information in Hardware
OSPF
Protocols
Unicast
Software
Unicast Routing
Tables Table
CEF
Hardware
H/W Adjacency
Table
Tables
PIM / IGMP
Multicast L2 Multicast
Multicast Routing
Table
MFIB
CEF – MFIB Subsystem
H/W FIB
Table
Chapter 37 Configuring IP Multicast
IGMP Snooping CPU
Spanning Tree
Subsystem
Software
Layer 2 Forwarding
Table
Integrated
MET Replication
Switching
Table
Engine
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