Layer 2 Switching Troubleshooting Basics

To understand why a switch is instrumental in today's networks, you need to have an understanding of many aspects of basic switching. You need to have a clear understanding of network traffic types, how broadcasts take away from the network bandwidth, the different media types in a network, and how switches overcome the problems inherent in bridges and switches. This section defines the following switch basics:

• Broadcast and unicast forwarding

• Aggregate bandwidth

• Full-duplex and half-duplex

• Supported media types

• Differences between bridges and switches

Broadcast and Unicast Forwarding

Knowing how switches and hubs treat broadcasts and unicasts can be an asset when you are troubleshooting switches. A general rule to remember about unicasts is that on a switch, unicasts are sent only to the destination port if the switch has learned the port the destination resides on. If the switch hasn't learned which port the destination resides on, the switch forwards the unicast out of all the ports that are members of the same virtual LAN (VLAN) as the source, with the exception of the port of arrival. A hub forwards unicasts and all data traffic out of its ports.

Broadcasts are sent to all the ports with the same VLAN membership as the receiving port, because everyone is the destination. A hub could care less about VLANs, and will send broadcasts to all of its ports.

Aggregate Bandwidth

Sometimes bottlenecks can develop in your network because the total number of ports that can bring data in equals more than the speed the trunk links can handle. For instance, a switch with 24 ports at 10Mbps each is capable of providing a total aggregate bandwidth of 240Mbps. If you have a 100Mbps trunk link between switches, the total amount of data coming from the ports can overwhelm the trunk link and cause a bottleneck. On a hub, the speed of the entire network is always equal to the speed of the physical media. For example, if you have a 10BaseT hub, the hub provides a total of 10Mbps of throughput.

Full-Duplex Versus Half-Duplex

Switches support half-duplex or full-duplex communications . Full-duplex refers to two-way communication that can take place simultaneously. A telephone call is an example of two-way communications. Both parties can speak and hear each other simultaneously .

Hubs support only half-duplex , which is communication that takes place on a single line. This can be similar to communication that takes place with a CB radio. The person holding down the button gets to speak while the other must listen. If both try to communicate simultaneously, a "collision" occurs.

Supported Media Types

The Catalyst 5000 and 4500 families of switches can support multiple mixed physical media types, such as Token Ring, Ethernet, Gigabit Ethernet, and Fiber Distributed Data Interface (FDDI), depending on the switch and the modules you have installed. The different modules in the switch provide the translations necessary between the different physical media types.

Differences Between Bridges and Switches

Some differences exist between bridges and switches and the types of physical media they support. Bridges and switches can provide support for mixed physical media environments, depending on the configuration and components installed on them. Switches process frames using application-specific integrated circuits (ASICs), which are chips that provide one or two tasks faster than a processor can. Bridges use software or generic hardware to provide processor tasks .

Another difference between bridges and switches is the number of ports each can support. Switches can provide ports that number in the hundreds. Bridges are typically 2- to 16-port devices.