Cisco Express Forwarding (CEF) uses a different kind of memory to
facilitate forwarding:
�¡ Uses TCAM
�¡ Topology-based switching (via Forwarding Information Base [FIB])
�¡ Can be centralized or distributed
Multilayer Switching
Multilayer Switching (MLS) is a switch feature that allows the switch to
route traffic between VLANs and routed interfaces in a highly optimized and
efficient manner. Cisco Express Forwarding (CEF) is an example technology
used to facilitate MLS (see Figure 4-1). Cisco Express Forwarding (CEF)
does the following:
Figure 4-1 Cisco Express Forwarding
BGP Table
Address Prefix AS-Path Next-Hop Communities Other Attr.
10.0.0.0 /8 42 13 1.2.3.4 37:12
... ... ... ... ... ...
IP Routing
Table
Address Prefix
... ...
FIB Table
(CEF Cache)
Protocol Address Next-Hop Outgoing Interface
BGP
ARP Cache
Adjacency Pointer
...
OSPF 1.2.3.0 1.5.4.1 Ethernet 0
MAC Address
...
IP Address
...
Layer 2 Header
...
Adjacency
Table
IP Address
...
1.5.4.1 MAC Header
Prefix
/24
Precedence
—
QoS Group
—
—
10.0.0.0 /8 1.2.3.4 — 3 7
BGP Table Map
Precedence
...
QoS Group
...
1.5.4.1 0c.00.11.22.33.44
10.0.0.0 /8 1.5.4.1 3 7
Conn. 1.5.4.0 /24 — Ethernet 0 —
■ Separates control plane hardware from data plane hardware.
■ Controls plane runs in software and builds FIB and adjacency table.
■ The data plane uses hardware to forward most IP unicast traffic.
■ Handles traffic that must be forwarded in software (much slower) and
includes:
—Packets originating from device.
—Packets with IP header options.
—Tunneled traffic.
—802.3 (IPX) frames.
—Load sharing traffic.
—FIB is an optimized routing table, stored in TCAM.
—Builds adjacencies from ARP data.
—Eliminates recursive loops.