CompTIA Network+ N10-009 Book

BGP (Border Gateway Protocol) is used for routing data between different ISPs, making it essential for the functioning of the internet. Its primary use is for exchanging routing information between autonomous systems, especially different ISPs. Preventing loops within a LAN ishandled by protocols like Spanning Tree Protocol (STP), while improving reconvergence times and sharing routes with a Layer 3 switch are functions of other protocols or internal mechanisms.

The best mitigation is implementing 802.1X, which provides port-based Network Access Control (NAC). With 802.1X enabled on access switch ports, a device plugged into a wall jack cannot gain normal network access until it successfully authenticates using credentials/certificates via an authentication server (commonly RADIUS). This directly addresses the threat of unauthorized users plugging into publicly accessible conference room jacks, because the switch keeps the port in an unauthenticated state (or places it into a restricted/guest VLAN) until authentication succeeds. This aligns with Network+ security objectives that emphasize controlling access at the edge, enforcing authentication, and reducing the risk of rogue or unmanaged devices on internal networks.

MAC filtering is weaker because MAC addresses can be spoofed and managing allow-lists at scale is error-prone. Creating a trusted zone is vague and does not prevent initial port access; segmentation helps limit blast radius but doesn’t enforce authentication at the jack. Disabling unused services is a general hardening practice, but it does not stop someone from connecting physically to an active switch port and attempting access. 802.1X is purpose-built for this exact scenario.

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The correct choice is B. Using a single public IP. PAT (Port Address Translation) is a form of NAT that allows many internal private IP addresses to share one public IP address by tracking sessions with port numbers. This is one of the most common edge-network designs for homes, small offices, and many enterprise branch environments.

The reason PAT is so useful is that it conserves public IPv4 addresses. Internal hosts keep their private addressing, and when they access external resources, the firewall or router translates those connections so they appear to come from one public address, while still keeping each session unique through port mapping.

The other options are unrelated to PAT’s primary purpose. BGP is a routing protocol for route exchange between networks. Redundant ISPs deal with resiliency and multihoming, not port-based address translation. A dual-stack IP scheme means running IPv4 and IPv6 together, which is separate from the NAT/PAT function.

This question is really asking what situation most naturally calls for PAT. When an organization wants multiple internal devices to reach the internet without assigning each device its own public IPv4 address, PAT is the standard solution. That makes using a single public IP the best answer.

The correct answer is A. The TX/RX connection is transposed. In fiber optic networking, communication requires proper alignment of the transmit (TX) and receive (RX) strands between devices. If these are reversed (not crossed correctly), the devices cannot properly send and receive signals, resulting in no link light. This is a common issue when working with fiber connections and media converters.

The scenario confirms that cables are tested and functional, and all devices are powered on, which eliminates physical damage or power issues. Fiber cables can still pass basic continuity tests while being incorrectly connected (TX to TX and RX to RX instead of TX to RX), which prevents link establishment.

Option B is incorrect because duplex mismatch (such as half vs. full duplex) typically causes performance issues like collisions and slow throughput, not a complete loss of link light. Option C is ruled out because the cables were tested successfully. Option D refers to errors that occur after a link is established, not a condition preventing link initialization.

According to CompTIA Network+ objectives, proper fiber polarity (TX/RX alignment) is critical for link establishment, making this the most accurate cause.