1492 Chapter 39
Ethernet switches receive a packet into memory.
They examine the destination address and decide which
of their ports has attached to it the station with the
address in question. Then if the destination is not on the
same port as the packet was received from, the switch
forwards the packet to only the correct port. Packets
where the destination address is on the same port as the
packet was received from are discarded.
Switches determine which addresses are connected
to each port by recording the source address of every
packet received, and associating that address with the
port from which it was received. This information is
assembled in a look up table (LUT) in the switch. Then
as each packet is received, the switch checks to see if
the destination address is in the LUT. If it is, the switch
knows where to send that packet and only sends it out
the appropriate port. If a destination address is not in the
LUT, the switch sends that packet out every port but the
one from which it was received. Since most Ethernet
stations respond to packets received addressed to them,
when the response is sent the switch learns which port
that address is on. Thereafter packets addressed to that
station are only sent out the correct port.
If a given MAC address is found on a different port
than was contained in the LUT, the LUT is corrected. If
no packet is received from a given MAC address within
a timeout window of perhaps 5 minutes, its entry in the
LUT is deleted. These characteristics allow the switch
to adapt and learn as network changes are made.
Packets intended to go out a given port are never
allowed to collide inside the switch. Instead each
outgoing packet is stored in a first in first out (FIFO)
buffer memory assigned to a given port, and transmitted
one at a time out the port.
While most data passing through a switch behaves as
described above, there is one type of packet that does
not. Most data packets are addressed to a specific desti-
nation MAC address. This is called unicast addressing.
There is a specific address called the multicast or broad-
cast address. Packets with this address in their destina-
tion field are sent to all stations. Therefore, these
packets are sent out all ports of a switch except the port
they came in on.
Switches are not the shared media of the early
coaxial cable Ethernet varieties, or the newer repeater
hubs. Instead by storing the packets, examining the
addresses, selectively passing the packets on, and FIFOFigure 39-28. Fiber used to interconnect two stars. This
allows breaking the copper cable 100 m limitation. Now
the only limitation is network diameter. As you can see
from Figs. 39-24, 39-25, and 39-26, quite large networks
can be made with simple wiring using the star of stars
approach. In all these examples, there are no loops. This is
because a loop in Ethernet, unless special techniques are
used, results in something called a broadcast storm, which
is sort of the data equivalent of runaway feedback in an
audio system.
Figure 39-29. Repeater hub functional diagram.
nodenodenodehub nodenodenodenodehub nodePort 1 Port 2 Port 3¾ ¾ ¾Figure 39-30. An Ethernet switch being used to isolate two
collision domains.nodenodenodehub nodenodenodenodehub nodeswitch