Information Technology Reference
In-Depth Information
A switch can physically replace a hub in your network. A switch allows multiple devices to be connected
to the same network, just like a hub does, but this is where the similarity ends. A switch allows each
connected device to have dedicated bandwidth instead of shared bandwidth. The bandwidth between the
switch and the device is reserved for communication to and from that device alone. Six devices
connected to six different ports on a 10-Mb switch would each have 10 Mb of bandwidth to work with,
instead of sharing that bandwidth with the other devices. A switch can greatly increase the available
bandwidth in your network, which can lead to improved network performance.
Bridges and Switches
A basic switch would be considered a Layer 2 device. When we use the word
layer
, we are referring to
the seven-layer OSI model. A switch does not just pass electrical signals along, like a hub does; instead,
it assembles the signals into a frame (Layer 2) and then decides what to do with the frame. A switch
determines what to do with a frame by borrowing an algorithm from another common networking device,
a transparent bridge. Logically, a switch acts just like a transparent bridge would, but it can handle
frames much faster than a transparent bridge (because of special hardware and architecture). When a
switch decides where the frame should be sent, it passes the frame out the appropriate port (or ports).
You can think of a switch as a device creating instantaneous connections between various ports, on a
frame-by-frame basis.
VLANs
Because the switch decides on a frame-by-frame basis which ports should exchange data, it is a natural
extension to put logic inside the switch to allow it to select ports for special groupings. This grouping of
ports is called a virtual local-area network (VLAN). The switch makes sure that traffic from one group
of ports never gets sent to other groups of ports (which would be routing). These port groups (VLANs)
can each be considered an individual LAN segment.
VLANs are also described as being broadcast domains. This is because of the transparent bridging
algorithm, which says that broadcast packets (packets destined for the “all devices” address) should be
sent out all ports that are in the same group (that is, in the same VLAN). Therefore, all ports that are in
the same VLAN are also in the same broadcast domain.
Transparent Bridging Algorithm
The transparent bridging algorithm and the Spanning-Tree Protocol are covered in more detail elsewhere
(see Chapter 20, “Troubleshooting Transparent Bridging Environments”). When a switch receives a
frame, it must decide what to do with that frame. It could ignore the frame, it could pass the frame out
one other port, or it could pass the frame out many other ports.
To know what to do with the frame, the switch learns the location of all devices on the segment. This
location information is placed in a CAM table (Content Addressable Memory, named for the type of
memory used to store these tables). The CAM table shows, for each device, the device's MAC address,
out which port that MAC address can be found, and which VLAN this port is associated with. The switch
continually does this
learning
process as frames are received into the switch. The switch's CAM table
is continually being updated.
This information in the CAM table is used to decide how a received frame should be handled. To decide
where to send a frame, the switch looks at the
destination
MAC address in a received frame and then
looks up that destination MAC address in the CAM table. The CAM table shows which port the frame
should be sent out for that frame to reach the specified destination MAC address.
