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Difficulty: HardInterface and Cable Troubleshooting Issues

Refer to the following Cisco IOS CLI output from a switch interface:

text
Switch# show interfaces gigabitEthernet 0/1
GigabitEthernet0/1 is up, line protocol is up (connected)
Hardware is GigabitEthernet, address is 0007.7d04.1a01
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Half-duplex, 100Mb/s, media type is 10/100/1000BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:01, 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 12000 bits/sec, 15 packets/sec
5 minute output rate 45000 bits/sec, 32 packets/sec
41205 packets input, 5142091 bytes, 0 no buffer
Received 104 broadcasts (0 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
58912 packets output, 7120491 bytes, 0 underruns
3412 output errors, 3290 late collisions, 0 deferred
0 lost carrier, 0 no carrier, 0 pause output

Based on the output, which condition is the most probable cause of the accumulated interface errors?

  1. The local interface is operating in half-duplex mode while the connected neighbor is hardcoded for full-duplex mode.Answer
  2. B
    An incorrect Ethernet cable pinout (straight-through instead of crossover) was used between the endpoints without Auto-MDIX.
  3. C
    The switch interface and connected router interface are configured with mismatched native VLAN IDs.
  4. D
    The CDP or LLDP protocol has been disabled on the remote device, causing misinterpretation of link capabilities.

Answer

The local interface is operating in half-duplex mode while the connected neighbor is hardcoded for full-duplex mode.
The combination of an 'up/up' link status, half-duplex local operational mode, and high late collision counts specifically indicates a duplex mismatch. When the remote device is manually set to full-duplex, auto-negotiation on the local device fails to detect duplex and falls back to half-duplex. As the full-duplex neighbor transmits at will without checking carrier sense, it interrupts ongoing local transmissions after the 64-byte collision window, causing late collisions to increment on the half-duplex side.

Step-by-Step Solution

1
Analyze line status and line protocol state.
Interface status is 'up' and line protocol is 'up', confirming physical Layer 1 connectivity and Layer 2 framing operational state.
This rules out physical disconnection or complete cable pinout failures.
2
Examine operating duplex settings and interface error counters.
The interface operates at 'Half-duplex, 100Mb/s' and shows significant 'late collisions' (3,290) and 'output errors' (3,412), but 0 CRC/input errors.
Late collisions occur when a collision is detected after the first 64 bytes (512 bits) of a frame have been transmitted.
3
Correlate symptoms with duplex operation mechanics.
Late collisions on a half-duplex interface are a textbook symptom of a duplex mismatch, occurring because the remote full-duplex node transmits while the local half-duplex node is midway through sending a frame.
In auto-negotiation fallback, if one side is set manually to full-duplex, the auto-negotiating side defaults to half-duplex, creating this exact symptom pattern.

Key Concept

Duplex Mismatch Identification via Interface Counters
Estimated Time:2m 0s
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Topics

Interface and Cable Troubleshooting Issues

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