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Difficulty: Very hardInterface and Cable Troubleshooting Issues

A network engineer is troubleshooting performance degradation on a link between Switch-A and Switch-B. The engineer executes the command `show interfaces fastEthernet 0/1` on Switch-A and observes the following CLI output:

text
FastEthernet0/1 is up, line protocol is up (connected)
Hardware is Fast Ethernet, address is 0007.eb78.8901 (bia 0007.eb78.8901)
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)
Full-duplex, 100Mb/s, media type is 100BaseTX
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
5 minute input rate 12000 bits/sec, 15 packets/sec
5 minute output rate 15000 bits/sec, 18 packets/sec
482910 packets input, 54910284 bytes, 0 no buffer
0 runts, 0 giants, 0 throttles
8942 input errors, 8942 CRC, 0 frame, 0 overrun, 0 ignored
512049 packets output, 61029481 bytes, 0 underruns
0 output errors, 0 collisions, 0 late collision, 0 deferred

Meanwhile, the connected interface on Switch-B reports `Half-duplex, 100Mb/s` and shows a rapidly incrementing counter for late collisions and deferred transmissions. Which of the following is the root cause of this troubleshooting issue?

  1. Switch-A is manually configured for full duplex while Switch-B relies on auto-negotiation, forcing Switch-B to default to half duplex.Answer
  2. B
    An incorrect crossover cable pinout is connected between Switch-A and Switch-B while auto-MDIX is disabled on both switches.
  3. C
    A native VLAN mismatch exists between the trunk ports of Switch-A and Switch-B, corrupting frame headers.
  4. D
    A corrupted Layer 2 discovery protocol TLV (CDP/LLDP) misinformed Switch-B about the link operational speed.

Answer

Switch-A is manually configured for full duplex while Switch-B relies on auto-negotiation, forcing Switch-B to default to half duplex.
The correct answer identifies a classic duplex mismatch scenario. When Switch-A is manually set to full duplex, it stops participating in auto-negotiation. Switch-B, left on auto-negotiation, can sense the 100 Mbps link speed via fast link pulses (FLP) but cannot negotiate the duplex setting. Following IEEE rules, Switch-B falls back to half duplex. Switch-A transmits whenever it has data, while Switch-B attempts CSMA/CD. This causes Switch-B to see late collisions and Switch-A to record CRC input errors.

Step-by-Step Solution

1
Analyze the interface output on Switch-A
Switch-A is operating at Full-duplex 100Mb/s, has 0 collisions/late collisions recorded, but shows a high number of input errors and CRC errors.
Full-duplex interfaces do not perform carrier sense multiple access with collision detection (CSMA/CD), so they never register collisions. However, incoming frames from a half-duplex partner transmitting during local transmissions will be truncated or corrupted, resulting in CRC errors.
2
Analyze the counter symptoms on Switch-B
Switch-B operates at Half-duplex and increments late collisions and deferred packets.
Because Switch-B is in half-duplex mode, it uses CSMA/CD. When Switch-A transmits continuously in full duplex, Switch-B detects collisions past the 64-byte slot time (late collisions).
3
Determine the cause of the duplex asymmetry
Disabling auto-negotiation on one end (hardcoding Switch-A to Full) causes the auto-negotiating peer (Switch-B) to fall back to the standard default of Half-duplex.
Cisco and IEEE 802.3 standards require auto-negotiation on both ends or manual configuration on both ends. Mixing hardcoded settings with auto-negotiation results in a duplex mismatch.

Key Concept

Ethernet Auto-Negotiation and Duplex Mismatch Troubleshooting
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Topics

Interface and Cable Troubleshooting Issues

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