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Difficulty: MediumUtilizing Command-Line Network Troubleshooting Utilities

A network administrator receives reports that a workstation at IP address 192.168.10.45 periodically loses connectivity to the default gateway. The administrator runs the `arp -a` command on the workstation and views the following output:

Interface: 192.168.10.45 --- 0x2
Internet Address Physical Address Type
192.168.10.1 00-11-22-33-44-55 dynamic
192.168.10.12 00-11-22-33-44-55 dynamic

Which of the following conditions is most directly indicated by this terminal output?

  1. A duplicate MAC address collision exists where another local IP address shares the gateway's physical address.Answer
  2. B
    The local DNS server failed to answer host resolution requests for the default gateway.
  3. C
    The workstation host is attempting to connect to the gateway across an mismatched TCP port number.
  4. D
    The workstation requires an AAAA resource record to populate dynamic entries in the IP cache.

Answer

A duplicate MAC address collision exists where another local IP address shares the gateway's physical address.
The `arp -a` utility displays the host's Address Resolution Protocol cache. Seeing two separate IP addresses (the gateway at 192.168.10.1 and a host at 192.168.10.12) bound to the identical physical MAC address (00-11-22-33-44-55) indicates a MAC collision, incorrect static assignment, or ARP spoofing, which leads to intermittent reachability issues to the default gateway.

Step-by-Step Solution

1
Analyze the output of the `arp -a` command.
Observe that IP address 192.168.10.1 (default gateway) and IP address 192.168.10.12 both resolve to MAC address 00-11-22-33-44-55.
The ARP table maintains mappings between Layer 3 IPv4 addresses and Layer 2 MAC addresses on the local network.
2
Evaluate the functional impact of identical physical addresses assigned to different logical IP addresses.
Frames destined for either IP address will be delivered to the same physical network interface card.
Unless running specialized multi-homing or virtual gateway clustering protocols, two distinct active IPv4 addresses should not dynamically resolve to the exact same MAC address on a local broadcast domain without causing frame misdirection or MAC address duplication conflicts.

Key Concept

Inspecting ARP Cache Entries to Identify Layer 2 / Layer 3 Addressing Conflicts
Estimated Time:1m 15s
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Topics

Utilizing Command-Line Network Troubleshooting Utilities

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