Receiving Data-Switching Modes

The first step in LAN switching is receiving the frame or packet, depending on the capabilities of the switch, from the transmitting device or host. Switches making forwarding decisions only at Layer 2 of the OSI model refer to data as frames, while switches making forwarding decisions at Layer 3 and above refer to data as packets. This chapter's examination of switching begins from a Layer 2 point of view. Depending on the model, varying amounts of each frame are stored and examined before being switched.

Three types of switching modes have been supported on Catalyst switches:

• Cut through

• Fragment free

• Store and forward

These three switching modes differ in how much of the frame is received and examined by the switch before a forwarding decision is made. The next sections describe each mode in detail.

Cut-Through Mode

Switches operating in cut-through mode receive and examine only the first 6 bytes of a frame. These first 6 bytes represent the destination MAC address of the frame, which is sufficient information to make a forwarding decision. Although cut-through switching offers the least latency when transmitting frames, it is susceptible to transmitting fragments created via Ethernet collisions, runts (frames less than 64 bytes), or damaged frames.

Fragment-Free Mode

Switches operating in fragment-free mode receive and examine the first 64 bytes of frame. Fragment free is referred to as "fast forward" mode in some Cisco Catalyst documentation. Why examine 64 bytes? In a properly designed Ethernet network, collision fragments must be detected in the first 64 bytes.

Store-and-Forward Mode

Switches operating in store-and-forward mode receive and examine the entire frame, resulting in the most error-free type of switching.

As switches utilizing faster processor and application-specific integrated circuits (ASICs) were introduced, the need to support cut-through and fragment-free switching was no longer necessary. As a result, all new Cisco Catalyst switches utilize store-and-forward switching.

Figure 2-1 compares each of the switching modes.