speed of 10 Mbps. Both the PC and the switch choose the fastest speed that each sup-
100BASE-T. The switch port and NIC negotiate to use the best speed of 100 Mbps and
NIC and switch port choose 1000 Mbps and full duplex.
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160 CCNA 200-301 Official Cert Guide, Volume 1
Autonegotiation Results When Only One Node Uses Autonegotiation
Figure 7-1 shows the IEEE autonegotiation results when both nodes use the process.
However, most Ethernet devices can disable autonegotiation, so it is just as important to
know what happens when a node tries to use autonegotiation but the node gets no response.
Disabling autonegotiation is not always a bad idea. For instance, many network engineers
disable autonegotiation on links between switches and simply configure the desired speed
and duplex on both switches. However, mistakes can happen when one device on an
Ethernet predefines speed and duplex (and disables autonegotiation), while the device on the
other end attempts autonegotiation. In that case, the link might not work at all, or it might
just work poorly.
NOTE
Configuring both the speed and duplex on a Cisco Catalyst switch interface dis-
ables autonegotiation.
IEEE autonegotiation defines some rules (defaults) that nodes should use as defaults when
autonegotiation fails—that is, when a node tries to use autonegotiation but hears nothing
from the device. The rules:
■
Speed: Use your slowest supported speed (often 10 Mbps).
■
Duplex: If your speed = 10 or 100, use half duplex; otherwise, use full duplex.
Cisco switches can make a better choice than that base IEEE speed default because Cisco
switches can actually sense the speed used by other nodes, even without IEEE autonegotia-
tion. As a result, Cisco switches use this slightly different logic to choose the speed when
autonegotiation fails:
■
Speed: Sense the speed (without using autonegotiation), but if that fails, use the IEEE
default (slowest supported speed, often 10 Mbps).
■
Duplex: Use the IEEE defaults: If speed = 10 or 100, use half duplex; otherwise, use full
duplex.
NOTE
Ethernet interfaces using speeds faster than 1 Gbps always use full duplex.
Figure 7-2 shows three examples in which three users change their NIC settings and disable
autonegotiation, while the switch (with all 10/100/1000 ports) attempts autonegotiation.
That is, the switch ports all default to speed auto and duplex auto. The top of the figure
shows the configured settings on each PC NIC, with the choices made by the switch listed
next to each switch port.
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Chapter 7: Configuring and Verifying Switch Interfaces 161
Manual Settings, Autonegotiation Disabled
Autonegotiation Enabled, 10/100/1000 Ports
1
2
3
10/100
10/100/1000
Settings:
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