To discuss some of the speed and duplex issues, first consider the output from the show
of these commands lists a one-line summary of the interface status, while the second com-
us more about autonegotiation.
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164 CCNA 200-301 Official Cert Guide, Volume 1
Example 7-7
Displaying Speed and Duplex Settings on Switch Interfaces
SW1# show interfaces status
Port Name Status Vlan Duplex Speed Type
Fa0/1 notconnect 1 auto auto 10/100BaseTX
Fa0/2 notconnect 1 auto auto 10/100BaseTX
Fa0/3 notconnect 1 auto auto 10/100BaseTX
Fa0/4 connected 1 a-full a-100 10/100BaseTX
Fa0/5 connected 1 a-full a-100 10/100BaseTX
Fa0/6 notconnect 1 auto auto 10/100BaseTX
Fa0/7 notconnect 1 auto auto 10/100BaseTX
Fa0/8 notconnect 1 auto auto 10/100BaseTX
Fa0/9 notconnect 1 auto auto 10/100BaseTX
Fa0/10 notconnect 1 auto auto 10/100BaseTX
Fa0/11 connected 1 a-full 10 10/100BaseTX
Fa0/12 connected 1 half 100 10/100BaseTX
Fa0/13 connected 1 a-full a-100 10/100BaseTX
Fa0/14 disabled 1 auto auto 10/100BaseTX
! Lines omitted for brevity
SW1# show interfaces fa0/13
FastEthernet0/13 is up, line protocol is up (connected)
Hardware is Fast Ethernet, address is 0019.e86a.6f8d (bia 0019.e86a.6f8d)
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mbps, media type is 10/100BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:05, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
85022 packets input, 10008976 bytes, 0 no buffer
Received 284 broadcasts (0 multicast)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 281 multicast, 0 pause input
0 input packets with dribble condition detected
95226 packets output, 10849674 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
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Chapter 7: Configuring and Verifying Switch Interfaces 165
0 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 PAUSE output
0 output buffer failures, 0 output buffers swapped out
Although both commands in the example can be useful, only the show interfaces status
command implies how the switch determined the speed and duplex settings. The command
output lists autonegotiated settings with a prefix of a- and the manually set values without
the a- prefix.
For example, consider ports Fa0/12 and Fa0/13 in the output of the show interfaces status
command. For Fa0/13, a-full means full duplex as autonegotiated, whereas half on Fa0/12
means half duplex but as manually configured. The example shades the command output
that implies that the switch’s Fa0/12 interface’s speed and duplex were not found through
autonegotiation, but Fa0/13 did use autonegotiation.
In comparison, note that the show interfaces fa0/13 command (without the status option)
simply lists the speed and duplex for interface Fast Ethernet 0/13, with nothing implying that
the values were learned through autonegotiation.
When the IEEE autonegotiation process works on both devices—that is, both are send-
ing autonegotiation messages—both devices agree to the fastest speed and best duplex
supported by both devices. However, when one device uses autonegotiation and the other
disables it, the first device must resort to default settings as detailed earlier in section
“Autonegotiation Results When Only One Node Uses Autonegotiation.” As a reminder,
those defaults are
■
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.
When a switch must use its defaults, it should get the speed correct, but it may choose the
wrong duplex setting, creating a duplex mismatch.
For example, in Figure 7-4, imagine that SW2’s Gi0/2 interface was configured with the
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