Which command is used on a Cisco router to enable IPv6 routing?
Correct Answer: B
Detailed Explanation: As networks transition from IPv4 to IPv6, it is essential that routers are configured to process IPv6 packets. By default, many Cisco routers do not have IPv6 routing enabled. The command “ipv6 unicast-routing” is used to activate IPv6 routing on a Cisco router, allowing the device to forward IPv6 packets between interfaces. What the Command Does: When you enter “ipv6 unicast-routing” in the global configuration mode, the router begins processing IPv6 routing information. This command enables the following functions: IPv6 Packet Forwarding: The router will examine IPv6 headers and make routing decisions based on IPv6 addresses. Neighbor Discovery and Auto-configuration: With IPv6 routing enabled, the router can participate in neighbor discovery (a critical function of IPv6) and support mechanisms like Stateless Address Autoconfiguration (SLAAC). Route Advertisement: The router can advertise IPv6 prefixes to other IPv6-enabled devices, either through routing protocols such as OSPFv3 or by sending Router Advertisement messages. Why This Command Is Important: Without enabling IPv6 routing, a Cisco router may recognize IPv6 addresses configured on interfaces but will not forward IPv6 traffic. This is particularly critical in dual-stack networks (networks that run both IPv4 and IPv6) where proper configuration ensures that both protocol families are correctly routed. Comparison with Other Options: Option A (ipv6 enable): Although some devices might support this command for enabling IPv6 on an interface, it does not globally enable IPv6 routing. Option C (ip ipv6-routing) and Option D (ipv6 routing): These commands are not recognized in standard Cisco IOS. The accepted command to globally enable IPv6 packet forwarding is “ipv6 unicast-routing”. Configuration Example: To enable IPv6 routing on a router, you would typically enter: Router# configure terminal Router(config)# ipv6 unicast-routing After this command is entered, any IPv6 addresses configured on interfaces will be used to build the IPv6 routing table, and the router will begin forwarding IPv6 traffic. Troubleshooting Considerations: If IPv6 traffic is not being forwarded as expected, one of the first configuration checks should be to verify whether “ipv6 unicast-routing” has been enabled. Without this command, even correctly configured IPv6 addresses will not result in proper packet forwarding, leading to connectivity issues. Conclusion: To enable IPv6 routing on a Cisco router, the correct command is “ipv6 unicast-routing” (Option B). This command is a fundamental step when deploying IPv6 in a network, ensuring that the router can properly forward IPv6 traffic and participate in IPv6 routing protocols. Understanding and applying this command is essential for the successful operation of dual-stack or IPv6-only environments.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.