Examkin

Question

Difficulty: MediumInterface and Cable Troubleshooting Issues

An administrator is configuring a direct point-to-point link between two router interfaces (Router1 GigabitEthernet0/0/0 and Router2 GigabitEthernet0/0/0) in a lab. Auto-MDIX has been manually disabled on both interfaces. After connecting the routers using a standard straight-through Ethernet cable, the administrator checks Router1 with the following command:

text
Router1# show interfaces gigabitEthernet 0/0/0
GigabitEthernet0/0/0 is down, line protocol is down (disabled)
Hardware is Gigabit Ethernet, address is 0050.56be.1a01 (bia 0050.56be.1a01)
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255

Which physical layer issue is the primary cause of both the interface status and line protocol remaining in a down/down state?

  1. An incorrect pinout cable type is used between two like MDI devices when Auto-MDIX is disabled.Answer
  2. B
    A native VLAN mismatch prevents Layer 2 frame transmission across the point-to-point link.
  3. C
    Layer 2 discovery protocols (CDP/LLDP) are failing to negotiate physical signaling parameters across the cable.
  4. D
    The router interfaces are misconfigured to combine two separate broadcast domains into a single collision domain.

Answer

The primary cause is the use of a straight-through cable between like devices (router to router) when Auto-MDIX is disabled, which requires a crossover cable to complete the physical circuit.
The correct answer identifies that connecting two like devices (router to router) without Auto-MDIX enabled requires a crossover cable. A straight-through cable maps transmit pins to transmit pins and receive pins to receive pins, which prevents physical carrier detection and leaves the link in a down/down state.

Step-by-Step Solution

1
Analyze the CLI interface state
The output shows 'GigabitEthernet0/0/0 is down, line protocol is down', indicating a Layer 1 physical connectivity issue.
When both interface and line protocol are down, physical signal electrical detection has failed.
2
Evaluate cabling requirements for like devices
Routers are MDI devices. Connecting MDI to MDI directly requires pin 1/2 (Tx) to connect to pin 3/6 (Rx).
A straight-through cable connects pin 1 to 1 and pin 2 to 2, causing both routers to transmit on the same pins and receive on the same pins.
3
Factor in the Auto-MDIX setting
Because Auto-MDIX is disabled, the interfaces cannot automatically adjust pin assignments electronically, requiring a physical crossover cable.
Without Auto-MDIX or a crossover cable, no link pulse/signal is detected, keeping the link down/down.

Key Concept

Ethernet Cable Pinouts and Auto-MDIX Operation
Rate this question

Topics

Interface and Cable Troubleshooting Issues

More questions from this topic