In many LANs, hubs and switches are
interconnected as a series. What this means is that the switches must
maintain MAC address not only for hosts that are directly connected to
one of its switch ports, but also MAC addresses of hosts connected to
other switches and hubs.
In Figure ,
Switch 1 and Switch 2 are interconnected. If Hosts A, B, C, and D send
information to Server H, Switch 2 will “learn” that these Hosts
(their MAC addresses) can all be reached via Switch 2’s port 1.
Figure ,
shows what Switch 2 SAT table would like. Remember, that if the switch
does not have the destination MAC address in its SAT table, then it
must flood the frame out all ports.
A completely switched network with all
switches (and hubs) is known as a “flat network.” A flat network
is a LAN made up entirely of hubs and switches, no routers. All hosts
on this LAN are on the same network or subnetwork. These networks are
easy to maintain, as there are no routers, so adding a new host or
other device is a relatively simple operation.
However, a flat network has several
disadvantages, including a single Layer 3 broadcast domain. As we will
see later, a Layer 2 broadcast like an ARP Request will travel to
every host and device on the LAN. These and other layer 2 broadcasts
can use up a great deal of a LANs available bandwidth.
There are other disadvantages as well,
including less manageability of network traffic and security. In a
flat network, once the switches have learned about what MAC addresses
are on which ports, the network traffic will flow accordingly. Except
for the placement of the switches, the network administrator has
little or no control over the path of the frames.
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