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Difficulty: MediumUtilizing Hardware Testers and Packet Analyzers

A network engineer is analyzing a packet capture (pcap) collected from a client workstation that is unable to authenticate with a RADIUS server. The trace shows the workstation sending UDP datagrams to port 1812 on the server IP address, followed immediately by ICMP Type 3, Code 3 (Destination Unreachable - Port Unreachable) response messages from the server. Which of the following conclusions is best supported by the packet analyzer output?

  1. The destination server host received the IP datagram, but no active RADIUS service is listening on the target UDP port, or a firewall actively rejected the port.Answer
  2. B
    The physical twisted-pair cable drop connecting the workstation has a split pair condition that is preventing Layer 1 signals from reaching the local switch.
  3. C
    The client application failed to establish communication because UDP requires a three-way SYN/SYN-ACK handshake before payload data can be transmitted.
  4. D
    The packet analyzer is unable to decode the datagram headers because UDP operates as a Layer 2 Data Link protocol.

Answer

The destination server host received the IP datagram, but no active RADIUS service is listening on the target UDP port, or a firewall actively rejected the port.
In packet capture analysis, receiving an ICMP Type 3 Code 3 (Port Unreachable) message directly from the target host proves that Layer 1 through Layer 3 network paths are functioning properly. The error explicitly confirms that the destination host received the packet, but no daemon or service was listening on UDP port 1812 (or an endpoint firewall explicitly blocked and rejected the port).

Step-by-Step Solution

1
Analyze the packet capture protocol types and control response codes.
Identified outgoing UDP packets addressed to destination port 1812 and returning ICMP Type 3, Code 3 packets originating from the target server.
ICMP Type 3 Code 3 specifically translates to 'Destination Unreachable: Port Unreachable'.
2
Correlate ICMP error feedback with OSI layer transport functionality.
Concluded that network connectivity up through Layer 3 is operational, but Layer 4 service binding failed on the target host.
An ICMP Port Unreachable message is generated by the destination host's network stack when an IP packet arrives for a port that has no active application listening on it.

Key Concept

Packet Analyzer Interpretation & ICMP Port Unreachable Response Diagnosis
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Topics

Utilizing Hardware Testers and Packet Analyzers

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