1496 Chapter 39
Not all managed switches will provide all the addi-
tional capabilities that will be mentioned. Check with
the manufacturer of the switch to determine its exact
capabilities.
39.11.9 Virtual Local Area Network (VLAN)
Virtual Local Area Network (VLAN) capability allows
certain switch ports to be isolated from the other ports.
This allows dividing up a larger switch into several
virtual smaller switches. While this capability may not
matter if all the data is unicast, if there is any multicast
or broadcast traffic, there might be significant benefit to
isolating that traffic to just certain ports. Some switches
may allow data from several VLANs to share a
common link to another switch without the data being
mingled. Both switches must support the same method
for doing this for it to work. Most today use a technique
called tagging to allow isolated VLANs to share a
common physical link between switches.
39.11.10 Quality of Service (QoS)
Quality of service (QoS) allows priority to be given to
certain data trying to leave a switch over other data. For
example, if we are sending audio over Ethernet using
CobraNet®, we would not want there to be any drop-
outs in the audio if there was a momentary spike in
normal computer data traffic through the switch. Such a
dropout could occur if a surge in computer data traffic
took up bandwidth needed for audio transmission and
delayed the reception of the audio packets.
Several means can be used to specify to the switch
which traffic is to be given priority. Priority can be
given to traffic on a certain VLAN, or that received
from certain ports, or that received from certain MAC
addresses, or even traffic containing a specific protocol
identifier.
39.11.11 Routing
Ethernet switches normally don’t examine the payload
portion of the Ethernet packet. Routers are capable of
looking inside the payload and routing Ethernet packets
based on Internet Protocol (IP) addresses that might be
found inside some Ethernet payloads. Such a router, or a
routing function built into some switches, can allow
packets to flow between normally independent
networks or VLANs. This can be very useful, for
example, to allow a central SNMP management system
to monitor and control all the network devices in a
facility even if they are in independent isolated
networks or VLANs.39.11.12 Fault Tolerance39.11.12.1 TrunkingTrunking or link aggregation allows two or more links
between switches to be combined to increase the band-
width between the switches. Both switches must
support trunking for this to work. While the algorithm
used to share the traffic between the links works for
many types of data, it does not for all possible types of
data. You may find situations where adding a second
link and activating trunking between two switches does
not provide any significant increase in available band-
width, Fig. 39-34.Trunking does provide increased fault tolerance,
particularly if the links aggregated run through different
physical paths between the two switches. If one link is
lost, the other link or links will continue to carry traffic
between the switches.39.11.12.2 Spanning TreeSpanning tree provides automatic protection from inad-
vertent loops in a network’s topology. The cost for this
protection is a delay in the activation of a connection
made to a port where spanning tree is activated while
the switch tries to determine if there is a path out thatFigure 39-34. Example of trunking between two switches.nodenodenodehub nodenodenodenodehub node