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Difficulty: HardInterface and Cable Troubleshooting Issues

Refer to the following Cisco IOS CLI output from switch Switch-A:

text
Switch-A# show interfaces fastEthernet 0/1
FastEthernet0/1 is up, line protocol is up (connected)
Hardware is Fast Ethernet, address is 0007.ebaa.1101
Full-duplex, 100Mb/s, media type is 100BASE-TX
...
12540 input errors, 12538 CRC, 0 frame, 0 overrun, 0 ignored
0 output errors, 0 collisions, 0 late collisions, 0 deferred

Switch-A connects directly to Switch-B over a 10-meter Category 5e UTP cable. Switch-B reports high counts of late collisions on its corresponding interface. Which condition is the root cause of these interface symptoms?

  1. A duplex mismatch where Switch-A is operating in full-duplex mode and Switch-B is operating in half-duplex mode.Answer
  2. B
    An incorrect Ethernet cable pinout where a rollover cable was installed instead of a straight-through cable.
  3. C
    A native VLAN mismatch where Switch-A is configured for Native VLAN 1 and Switch-B is configured for Native VLAN 10.
  4. D
    A Layer 2 neighbor discovery mismatch where Switch-A failed to parse CDP network interface Type-Length-Values (TLVs).

Answer

The root cause is a duplex mismatch where Switch-A is set to full-duplex and Switch-B is set to half-duplex.
A duplex mismatch occurs when one end of an Ethernet link operates in full-duplex mode while the opposite end operates in half-duplex mode. Because the full-duplex device (Switch-A) transmits data without performing carrier sensing, it frequently sends frames while the half-duplex device (Switch-B) is actively transmitting. This causes Switch-B to detect collisions after sending the initial 64 bytes of a frame (late collisions), while Switch-A receives aborted frame fragments from Switch-B, registering them as CRC and FCS input errors.

Step-by-Step Solution

1
Analyze the CLI output on Switch-A
The interface is operating in full-duplex mode with a high number of input errors dominated by CRC errors, while output errors and collisions remain at zero.
An interface configured for full-duplex never checks for carrier sensing or collisions, so its collision counters will always remain zero.
2
Analyze the reported symptoms on Switch-B
Switch-B reports high counts of late collisions.
Late collisions occur on half-duplex interfaces when another device transmits after the 64-byte slot time window has passed.
3
Correlate symptoms across both devices
Full-duplex on Switch-A + Half-duplex on Switch-B explains both CRC errors on Switch-A and late collisions on Switch-B.
Switch-A transmits whenever it has data without listening. When Switch-B is transmitting, Switch-A's concurrent transmission collides with Switch-B's frames, causing Switch-B to record late collisions and Switch-A to receive truncated/corrupted frame fragments resulting in CRC errors.

Key Concept

Duplex Mismatch Symptom Analysis
Estimated Time:2m 0s
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Topics

Interface and Cable Troubleshooting Issues

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