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Chapter 1

Internetworking

Bridges and switches read each frame as it passes through the network.

The layer-2 device then puts the source hardware address in a filter table and
keeps track of which port the frame was received on. This information
(logged in the bridge's or switch's filter table) is what helps the machine
determine the location of the specific sending device.

The real estate business is all about location, location, location, and it's

the same way for both layer-2 and -3 devices. Though both need to be able
to negotiate the network, it's crucial to remember that they're concerned
with very different parts of it. Routers, or layer-3 machines, need to locate
specific networks, whereas layer-2 machines (switches and bridges) need to
locate specific devices. So, networks are to routers as individual devices are
to switches and bridges. And routing tables that "map" the internetwork are
for routers, as filter tables that "map" individual devices are for switches and
bridges.

After a filter table is built on the layer-2 device, it will only forward frames

to the segment where the destination hardware address is located. If the des-
tination device is on the same segment as the frame, the layer-2 device will
block the frame from going to any other segments. If the destination is on a
different segment, the frame can only be transmitted to that segment. This is
called transparent bridging.

When a switch interface receives a frame with a destination hardware

address that isn't found in the device's filter table, it will forward the frame
to all connected segments. If the unknown device that sent the "mystery
frame" replies to this forwarding action, the switch updates its filter table
regarding that device's location. But in the event the destination address of
the transmitting frame is a broadcast address, the switch will forward all
broadcasts to every connected segment by default.

All devices that the broadcast is forwarded to are considered to be in the

same broadcast domain. This can be a problem; layer-2 devices propagate
layer-2 broadcast storms that choke performance, and the only way to stop
a broadcast storm from propagating through an internetwork is with a layer-3
device--a router.

The biggest benefit of using switches instead of hubs in your internetwork

is that each switch port is actually its own collision domain. (Conversely, a
hub creates one large collision domain.) But even armed with a switch, you
still can't break up broadcast domains. Neither switches nor bridges will do
that. They'll simply forward all broadcasts instead.

Another benefit of LAN switching over hub-centered implementations is

that each device on every segment plugged into a switch can transmit

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