Layer 2 LAN Switching
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However, as shown in Table 4.1, there are some differences between
bridges and switches that you should be aware of:
Bridges are considered software based, and switches are hardware
based because they use an ASIC's chip to help make filtering decisions.
Bridges can have only one spanning-tree instance per bridge, and
switches can have many. (Spanning tree is covered later in this chapter.)
Bridges can have only up to 16 ports, whereas a switch can have
hundreds.
You probably won't go out and buy a bridge, but it's important to under-
stand how bridges are designed and maintained because layer 2 switches
function in a similar fashion.
Three Switch Functions at Layer 2
There are three distinct functions of layer 2 switching:
Address learning Layer 2 switches and bridges remember the source
hardware address of each frame received on an interface and enter it into
a MAC database.
Forward/filter decision When a frame is received on an interface, the
switch looks at the destination hardware address and looks up the exit
interface in the MAC database.
Loop avoidance If multiple connections between switches are created
for redundancy, network loops can occur. STP is used to stop network
loops and allow redundancy.
T A B L E 4 . 1
Comparison of Bridges and Switches
Bridges
Switches
Filtering
Software based
Hardware based
Spanning tree
numbers
One spanning tree
instance
Many spanning tree
instances
Ports
16 ports maximum
Hundreds of ports available
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