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Switching Services

Plus, layer-2 switching increases bandwidth for each user because, again,

each connection (interface) into the switch is its own collision domain.
This feature makes it possible for you to connect multiple devices to each
interface.

Limitations of Layer-2 Switching

Since we commonly stick layer-2 switching into the same category as bridged
networks, we also tend to think it has the same hang-ups and issues that
bridged networks do. Keep in mind that bridges are good and helpful things
if we design the network correctly, keeping their features as well as their lim-
itations in mind. And to design well with bridges, the two most important
considerations are:

We absolutely must break up the collision domains correctly.

The right way to create a functional bridged network is to make sure
that its users spend 80 percent of their time on the local segment.

Bridged networks break up collision domains, but remember, that network is

still one large broadcast domain. Both layer-2 switches and bridges don't break
up broadcast domains by default--something that not only limits your net-
work's size and growth potential, but can also reduce its overall performance.

Broadcasts and multicasts, along with the slow convergence time of span-

ning trees, can give you some major grief as your network grows. These are
the big reasons why layer-2 switches and bridges cannot completely replace
routers (layer-3 devices) in the internetwork.

Bridging vs. LAN Switching

It's true--layer-2 switches really are pretty much just bridges that give us a
bunch more ports, but there are some important differences you should
always keep in mind:

Bridges are software based, while switches are hardware based
because they use ASIC chips to help make filtering decisions.

Bridges can only have one spanning-tree instance per bridge, while
switches can have many. (I'm going to tell you all about spanning trees
in a bit.)

Switches have a higher number of ports than most bridges.

Both bridges and switches forward layer-2 broadcasts.

Both bridges and switches make forwarding decisions based on layer-2
addresses.

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