Layer 2 switching forwards frames based on information in the Layer
2 Frame header as shown in the figure. Layer 2 switching occurs in hardware
thereby decreasing latency introduced by software switching typically found in
original bridge platforms. Switch hardware utilizes specialized chips, called
application-specific integrated circuits (ASICs), to handle frame manipulation
and forwarding. Because the majority of frame manipulation and forwarding
decisions occur in hardware, Layer 2 switching can provide wire-speed
performance in ideal circumstances.
A Layer 2 switch builds a forwarding
table as it records the source MAC address and the inbound port number of
received frames. Because the switch simply moves frames from one port to
another, based on the information in the forwarding table, operation is said to
be transparent; the sending end station is unaware of the switch path traversed
by the frame.

Additionally, the frame can be checked against access control list (ACL) and
quality of service (QoS) criteria that originate in Layer 3 software, but are
stored in tables in switch hardware, to facilitate wire-speed lookups. This
process provides frame forwarding at wire-speed while still qualifying the
forwarding based on upper layer criteria.
What are Layer 2 Switching
Tables?
Routing, switching, ACL and QoS tables are stored in a
high-speed table memory so that forwarding decisions and restrictions can be
made in high-speed hardware. Cisco Catalysts have two primary table
architectures:

-
CAM Table – content addressable memory table. This is the primary
table used to make Layer 2 forwarding decisions. The table is built by
recording the source address and inbound port of all frames. When a frame
arrives at the switch with a destination MAC address of an entry in the CAM
table, the frame is forwarded out only the port associated with that specific
MAC address.
-
TCAM Table – tertiary CAM table. This table stores ACL, QoS and
other information generally associated with upper layer processing.
Table lookups are done with efficient search algorithms. A
"key" is created to compare the frame to the table content. For
example, the destination MAC address and VLAN ID (VID) of a frame would
constitute the key for Layer 2 table lookup. This key is fed into a hashing
algorithm, which produces a pointer into the table. The system uses the pointer
to access a smaller specific area of the table without requiring searching the
entire table.
In a Layer 2 table, all bits of all information are significant for frame
forwarding (for example, VLANs, destination MAC addresses, and destination
protocol types). However, in more complicated tables associated with upper
layer forwarding criteria, some bits of information may be inconsequential to
analyze. For example, an ACL may require a match on the first 24 bits of an IP
address but the last 8 bits are insignificant information.
Identifying the Layer 2 Switch Forwarding Process
Layer 2
forwarding in hardware is based on the destination MAC address.
The Layer 2
switch learns the address based on the source MAC address. The MAC address
table lists MAC and VLAN pairs with associated interfaces.
