NOTE
The show running-config and show startup-config commands typically do not dis-
play default configuration settings, so the absence of commands listed under interface F0/2
at the end of the example means that those commands now use default values.
Autonegotiation
For any 10/100 or 10/100/1000 interfaces—that is, interfaces that can run at different
speeds—Cisco Catalyst switches default to a setting of duplex auto and speed auto. As a
result, those interfaces attempt to automatically determine the speed and duplex setting to
use. Alternatively, you can configure most devices, switch interfaces included, to use a spe-
cific speed and/or duplex.
In practice, using autonegotiation is easy: just leave the speed and duplex at the default set-
ting, and let the switch port negotiate what settings to use on each port. However, problems
can occur due to unfortunate combinations of configuration. Therefore, this next topic walks
through more detail about the concepts behind autonegotiation, so you know better how to
interpret the meaning of the switch show commands and when to choose to use a particular
configuration setting.
Autonegotiation Under Working Conditions
Ethernet devices on the ends of a link must use the same standard; otherwise, they can-
not correctly send data. For example, a NIC cannot use 100BASE-T, which uses a two-pair
UTP cable with a 100-Mbps speed, while the switch port on the other end of the link uses
1000BASE-T. Even if you used a cable that works with Gigabit Ethernet, the link would not
work with one end trying to send at 100 Mbps while the other tried to receive the data at
1000 Mbps.
Upgrading to new and faster Ethernet standards becomes a problem because both ends have
to use the same standard. For example, if you replace an old PC with a new one, the old one
might have been using 100BASE-T while the new one uses 1000BASE-T. The switch port on
the other end of the link needs to now use 1000BASE-T, so you upgrade the switch. If that
switch had ports that would use only 1000BASE-T, you would need to upgrade all the other
PCs connected to the switch. So, having both PC network interface cards (NIC) and switch
ports that support multiple standards/speeds makes it much easier to migrate to the next
better standard.
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Chapter 7: Configuring and Verifying Switch Interfaces 159
The IEEE autonegotiation protocol helps makes it much easier to operate a LAN when NICs
and switch ports support multiple speeds. IEEE autonegotiation (IEEE standard 802.3u)
defines a protocol that lets the two UTP-based Ethernet nodes on a link negotiate so that
they each choose to use the same speed and duplex settings. The protocol messages flow
outside the normal Ethernet electrical frequencies as out-of-band signals over the UTP cable.
Basically, each node states what it can do, and then each node picks the best options that
both nodes support: the fastest speed and the best duplex setting, with full duplex being
better than half duplex.
NOTE
Autonegotiation relies on the fact that the IEEE uses the same wiring pinouts for
10BASE-T and 100BASE-T, and that 1000BASE-T simply adds to those pinouts, adding two
pairs.
Many networks use autonegotiation every day, particularly between user devices and the
access layer LAN switches, as shown in Figure 7-1. The company installed four-pair cabling
of the right quality to support 1000BASE-T, to be ready to support Gigabit Ethernet. As a
result, the wiring supports 10-Mbps, 100-Mbps, and 1000-Mbps Ethernet options. Both
nodes on each link send autonegotiation messages to each other. The switch in this case has
all 10/100/1000 ports, while the PC NICs support different options.
Autonegotiation Enabled
Autonegotiation Enabled, 10/100/1000 Ports
1
2
3
10
10/100
Result:
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