This location information is placed in a Content Addressable Memory table (CAM - named for the type of memory used to store these tables). In order to know what to do with the frame, the switch learns the location of all devices on the segment. It could ignore the frame it could pass the frame out one other port, or it could pass the frame out many other ports. When a switch receives a frame, it must decide what to do with that frame. The transparent bridging algorithm and spanning tree are covered in more detail elsewhere (Chapter 20: Troubleshooting Transparent Bridging Environments). All ports that are in the same VLAN are also in the same broadcast domain. This is because of the transparent bridging algorithm, which says that broadcast packets (packets destined for the all devices address) be sent out all ports that are in the same group (that is, in the same VLAN). VLANs are also described as broadcast domains. These port groups (VLANs) can each be considered an individual LAN segment. The switch makes sure that traffic from one group of ports never gets sent to other groups of ports (which would be routing). This grouping of ports is called a Virtual Local Area Network (VLAN). Since the switch decides on a frame by frame basis which ports exchange data, it is a natural extension to put logic inside the switch to allow it to choose ports for special groupings. You can think of a switch as a device creating instantaneous connections between various ports, on a frame by frame basis. Once a switch decides where the frame should be sent, it passes the frame out the appropriate port (or ports). Logically, a switch acts just like a transparent bridge would, but it can handle frames much faster than a transparent bridge could (because of special hardware and architecture). A switch determines what to do with a frame by borrowing an algorithm from another common networking device: a transparent bridge. A switch does not just pass electrical signals along, like a hub does instead, it assembles the signals into a frame (layer two), and then decides what to do with the frame. When we use the word layer, we are referring to the 7-layer OSI model. Bridges and SwitchesР”Р„ basic switch is considered a layer-two device. A switch can greatly increase the available bandwidth in your network, which can lead to improved network performance. Six devices connected to six different ports on a 10Mb switch each have 10Mb of bandwidth to work with, instead of shared bandwidth with the other devices. The bandwidth between the switch and the device is reserved for communication to and from that device alone. A switch allows each connected device to have dedicated bandwidth instead of shared bandwidth. A switch allows multiple devices to be connected to the same network, just like a hub does, but this is where the similarity ends. It hears an electrical signal on the wire and passes it along to the other ports.Р”Р„ switch can physically replace a hub in your network. In terms of the OSI model, a hub is considered a layer-one (physical layer) device. A 100Mb hub shares 100Mb of bandwidth among the connected devices. If it is a 10Mb hub, and there are 6 devices connected to 6 different ports on the hub, all six devices share the 10Mb of bandwidth with each other. The devices on that segment share the bandwidth with each other.

A hub allows multiple devices to be connected to the same network segment. Hubs and SwitchesР”В«ecause of the great demand placed on local area networks, we have seen a shift from a shared bandwidth network, with hubs and coaxial cable, to a dedicated bandwidth network, with switches.

These paragraphs describe some of the key concepts to know about switches. Switches have become much more complex over the last few years because they have gained in popularity and sophistication.

One of the prerequisites to troubleshooting any device is to know the rules under which it operates. If you are new to LAN switching, these sections take you through some of the main concepts related to switches. Refer to Cisco Technical Tips Conventions for more information on document conventions. There are no specific requirements for this document. Using Portfast and other commands to fix end-station startup connectivity problemsР”В¬onfiguring and troubleshooting multilayer switching ISL trunking on Catalyst 50 family switchesР”В¬onfiguring and troubleshooting EtherChannel switch to switch Troubleshooting Ethernet 10/100Mb half/full duplex auto-negotiation Troubleshooting port connectivity problems

General switch troubleshooting suggestions The sections in this chapter describe common LAN switch features and solutions to some of the most common LAN switching problems.

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