Examkin

Question

Difficulty: HardInterface and Cable Troubleshooting Issues

A network engineer is troubleshooting performance degradation on switch interface FastEthernet0/1. The output of the `show interfaces fastethernet 0/1` command is shown below:

text
FastEthernet0/1 is up, line protocol is up (connected)
Hardware is FastEthernet, address is 0019.e762.a801 (bia 0019.e762.a801)
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 100BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:02, output 00:00:01, 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 32000 bits/sec, 28 packets/sec
5 minute output rate 88000 bits/sec, 64 packets/sec
521098 packets input, 68191044 bytes, 0 underruns
Received 42 broadcasts (0 multicasts)
0 runts, 0 giants, 0 throttles
12480 input errors, 12472 CRC, 8 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
889123 packets output, 118129033 bytes, 0 underruns
0 output errors, 38102 collisions, 9841 late collision, 0 deferred

Based on the CLI output, which two underlying issues could be causing the observed interface counters? (Select TWO.)

  1. A duplex mismatch where the local interface is operating in half-duplex while the connected endpoint is operating in full-duplexAnswer
  2. The Ethernet cable length exceeds the maximum 100-meter distance limit defined by Category 5e/6 specificationsAnswer
  3. C
    An incorrect Ethernet cable pinout (straight-through instead of crossover) connecting two devices without Auto-MDIX support
  4. D
    A native VLAN ID mismatch configured between opposing switch trunk ports

Answer

The two probable causes are a duplex mismatch (where the local interface is operating in half-duplex while the remote end is in full-duplex) and an Ethernet cabling run that exceeds the maximum allowed 100-meter distance specification.
Late collisions are defined as collisions occurring after the first 64 bytes (512 bits) of a frame have been transmitted. The two primary causes of late collisions on an Ethernet link are: 1) A duplex mismatch where one end operates in half-duplex and the opposing end operates in full-duplex (the full-duplex side transmits without listening, interrupting ongoing transmissions after 64 bytes), and 2) Physical cable lengths exceeding the maximum specification (100 meters for UTP), which causes propagation delays longer than the 512-bit slot time.

Step-by-Step Solution

1
Analyze the interface operating mode and error counters from the CLI output
The interface is set to Half-duplex mode at 100 Mbps and shows significant late collision counters (9,841) and CRC errors (12,472).
Late collisions occur when a collision is detected after the first 64 bytes (512 bits) of a frame have already been transmitted.
2
Evaluate duplex mismatch mechanics
A full-duplex endpoint transmits at any time without performing carrier sensing, sending frames that collide with the half-duplex interface after its 64-byte transmission window has passed.
The full-duplex device ignores half-duplex collision domain rules, causing late collisions on the half-duplex receiver.
3
Evaluate physical cable distance limits
Exceeding the 100-meter UTP distance limit increases signal propagation delay beyond the standard Ethernet 512-bit slot time.
When propagation delay exceeds the slot time, normal collisions arrive too late to be handled as standard collisions, incrementing the late collision counter instead.

Key Concept

Late collisions on Ethernet interfaces are primarily caused by duplex mismatches or physical cable length limit violations (>100m).
Estimated Time:2m 0s
Rate this question

Topics

Interface and Cable Troubleshooting Issues

More questions from this topic