Which protocol is used by IPv6 devices to determine the link-layer addresses of neighboring devices on the same network segment?
Correct Answer: B
Detailed Explanation: In IPv4 networks, the Address Resolution Protocol (ARP) is used to map IP addresses to MAC (link-layer) addresses. However, IPv6 does not use ARP. Instead, IPv6 employs the Neighbor Discovery Protocol (NDP), which is defined as part of the Internet Control Message Protocol for IPv6 (ICMPv6). NDP is essential for several functions in an IPv6 network, including address resolution, router discovery, prefix discovery, and neighbor unreachability detection. How NDP Works: When an IPv6-enabled device needs to determine the link-layer address associated with a specific IPv6 address on its local link, it sends a Neighbor Solicitation message. This message is multicast to the solicited-node multicast address corresponding to the target IPv6 address. Devices on the local link that have the matching IPv6 address respond with a Neighbor Advertisement message, which includes their link-layer (MAC) address. This process allows devices to build and maintain a neighbor cache—a table similar in function to an ARP cache—that stores the mapping between IPv6 addresses and MAC addresses. Key Functions of NDP: Address Resolution: NDP replaces ARP by enabling devices to resolve IPv6 addresses to MAC addresses. Router Discovery: Hosts use NDP to discover the presence of routers on the local link. Routers periodically send Router Advertisement messages, which include network prefixes and other configuration information. Prefix Discovery: Devices learn about the network prefixes used on the link, which aids in auto-configuration. Neighbor Unreachability Detection: NDP helps determine whether a neighbor is still reachable. If a device does not receive a response to its Neighbor Solicitation, it can mark the neighbor as unreachable and initiate recovery procedures. Why ARP Is Not Used in IPv6: ARP was designed for IPv4 and is not scalable for IPv6’s vastly expanded address space. NDP uses ICMPv6 messages that are more flexible and secure than ARP, and it integrates seamlessly with other IPv6 functionalities, such as Stateless Address Autoconfiguration (SLAAC). This integration enables IPv6 devices to automatically configure themselves and learn about network parameters without relying on a separate ARP mechanism. Security Considerations: While NDP is powerful, it also introduces new security challenges. For instance, attackers can exploit NDP through spoofing attacks. To mitigate such risks, enhancements like Secure Neighbor Discovery (SEND) have been proposed, though SEND is not yet widely implemented. Network administrators must be aware of these potential vulnerabilities and use additional security measures (such as IPsec) to protect the network. Conclusion: Neighbor Discovery Protocol (NDP) is the correct answer because it provides the mechanism by which IPv6 devices resolve link-layer addresses, discover routers, and maintain neighbor reachability—all functions that ARP performed in IPv4. Understanding NDP is crucial for anyone working with IPv6, as it underpins many of the protocol’s automatic configuration and network management features.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.