IPv6 Configuration Scenarios

Change R1 G0/0 subnet mask to /30, add a default route via 192.0.2.254, and keep the EUI-64 IPv6 address on G0/0.

Why wrong: This is incorrect because the default route points to 192.0.2.254, which is not R2's G0/0 IP (192.0.2.2), so IPv4 traffic will not be forwarded. Also, EUI-64 generates an interface ID from the MAC, which will not match the subnet expected by R2's static address 2001:db8:1::2/64.

Change R1 G0/0 subnet mask to /24, add a default route via 192.0.2.2, and configure a static IPv6 address 2001:db8:1::1/64 on G0/0.

Why wrong: This is incorrect because the subnet mask on R1's G0/0 remains /24, which does not match R2's /30. R1 will consider the subnet to be 192.0.2.0/24, while R2 uses 192.0.2.0/30, causing a mismatch that prevents direct communication.

Change R1 G0/0 subnet mask to /30, add a default route via 192.0.2.2, and keep the EUI-64 IPv6 address on G0/0.

Why wrong: This is incorrect because while the IPv4 issues are fixed, the IPv6 address generated by EUI-64 will not match the subnet expected by R2's static address 2001:db8:1::2/64. EUI-64 creates an interface ID based on the MAC, which is unpredictable and unlikely to be in the same subnet as R2's static address.

A floating static route is configured with a lower administrative distance than the primary route.

Why wrong: A floating static route must have a higher administrative distance than the primary route so it is only used as a backup.

A static route with an administrative distance of 1 is preferred over a directly connected route.

Why wrong: Directly connected routes have an administrative distance of 0, which is lower (more preferred) than 1.

IPv4 and IPv6 static routes are configured using the same command syntax.

Why wrong: IPv4 static routes use 'ip route' while IPv6 static routes use 'ipv6 route'. They are different commands.

The OSPFv3 process ID must match on both routers.

Why wrong: OSPFv3 process IDs are locally significant and do not need to match for adjacency to form.

The link-local addresses are not in the same subnet.

Why wrong: Link-local addresses are only used for neighbor discovery and do not require subnet matching; they are always on the same link.

The router ID 1.1.1.1 is duplicated on R2.

Why wrong: Duplicate router IDs can cause issues, but the 'show ospfv3 neighbor' output would typically show the neighbor in EXSTART or EXCHANGE state, not missing entirely.

Unique local

Why wrong: Unique local addresses are not automatically the local-link-only address being tested here.

Global unicast

Why wrong: Global unicast addresses are routable beyond the local link.

Anycast

Why wrong: Anycast is an addressing behavior, not the specific auto-created local type.

The routers need matching hostnames before OSPFv3 can start.

Why wrong: This is wrong because hostnames do not determine OSPFv3 adjacency formation.

IPv6 requires a /64 only for routing protocols to function.

Why wrong: This is wrong because the specific adjacency issue here is best explained by a protocol mismatch, not a blanket statement about all IPv6 routing.

OSPFv3 cannot run on directly connected interfaces.

Why wrong: This is wrong because OSPFv3 is designed to run on directly connected interfaces.

ipv6 enable

Why wrong: ipv6 enable is used at the interface level to enable IPv6 processing, not global routing.

ip routing ipv6

Why wrong: This is not valid Cisco IOS syntax.

ipv6 route enable

Why wrong: This is not a valid IOS command.

Add a static route to 10.0.0.0/30 via the backup link to R3, then the default route to 10.0.0.2 will work. Then configure IPv6 default routes with AD 1 and 10 as described.

Why wrong: This is incorrect because adding a static route to 10.0.0.0/30 via the backup link would cause the default route to use the backup path, defeating the purpose of having a primary path. The issue is that the next-hop 10.0.0.2 is not directly connected or reachable; the correct fix is to ensure the interface is up and the next-hop is on a directly connected subnet, not to add a static route via the backup.

Change the administrative distance of the IPv4 default route to 10 and the backup to 1, so the backup becomes primary. Then configure IPv6 default routes with AD 10 and 1 respectively.

Why wrong: This is incorrect because it swaps the intended primary and backup paths. The requirement specifies that the primary path goes through R2 with AD 1 and backup through R3 with AD 10. Changing the AD values would make R3 the primary, which is not desired.

Remove the existing IPv4 static default route and configure it with the next-hop as the exit interface (e.g., GigabitEthernet0/0) instead of the IP address. Then add IPv6 default routes using the exit interface as well.

Why wrong: This is incorrect because using an exit interface for a static route (point-to-point) is valid, but it does not fix the recursive routing failure if the next-hop IP is unreachable. The issue is likely that the next-hop IP is not in the routing table; using the interface might bypass the recursive lookup but would still require the interface to be up. However, the requirement specifies using next-hop IP addresses, and the IPv6 routes also need next-hop IPs. Moreover, the problem statement indicates the next-hop is 10.0.0.2, which should be directly connected; using the interface is not necessary and may not match the intended design.

DHCPv6

Why wrong: DHCPv6 provides addressing information, not local-link address resolution by itself.

EUI-64

Why wrong: EUI-64 is one method of generating an interface identifier, not an address resolution protocol.

SLAAC

Why wrong: SLAAC is an addressing method, not the ARP replacement mechanism.

FC00::/7

Why wrong: FC00::/7 is unique local addressing, not link-local.

2000::/3

Why wrong: 2000::/3 is global unicast space.

FF00::/8

Why wrong: FF00::/8 is multicast space.

They are globally routable across the Internet.

Why wrong: This is wrong because link-local addresses are not Internet-routable.

They exist only when DHCPv6 fails.

Why wrong: This is wrong because link-local addresses are standard and normal in IPv6.

They replace the need for any default gateway logic.

Why wrong: This is wrong because hosts still rely on gateway behavior for off-link communication.

The IPv6 default gateway can be any global unicast address on the internet.

Why wrong: The IPv6 default gateway must be on the same link as the host (typically the link-local address of the router), not any global unicast address.

The subnet mask is used to define the host portion of an IPv4 address.

Why wrong: The subnet mask separates the network and host portions; it does not determine the host portion—it defines the boundary.

Configuring a DNS server is mandatory for a host to communicate with any other device on the same subnet.

Why wrong: DNS is used for name resolution, not for basic IP connectivity. Devices can communicate using IP addresses without DNS.

R1(config-if)# ipv6 address fe80::/10 eui-64 R1(config-if)# ipv6 address 2001:db8:1::/64 eui-64 R1(config-if)# no shutdown

Why wrong: This is incorrect because the link-local address cannot be manually configured with the eui-64 keyword in this manner; it is automatically generated by the ipv6 enable command. The command ipv6 address fe80::/10 eui-64 is not valid for link-local addresses.

R1(config-if)# ipv6 address 2001:db8:1::/64 eui-64 R1(config-if)# no shutdown

Why wrong: This is incorrect because without the ipv6 enable command, the interface will not generate a link-local address, which is required for IPv6 operation. The global unicast address alone does not enable IPv6 on the interface.

R1(config-if)# ipv6 enable R1(config-if)# ipv6 address 2001:db8:1::1/64 R1(config-if)# no shutdown

Why wrong: This is incorrect because the global unicast address is configured with a static interface ID (::1) instead of using EUI-64 as required by the question. The question specifies the use of EUI-64 for the global unicast address.

It changes the /64 prefix into a /48 prefix for summarization.

Why wrong: This is wrong because EUI-64 does not alter the network prefix length.

It replaces the need for a link-local address.

Why wrong: This is wrong because link-local addresses are still a normal and important part of IPv6.

It encrypts IPv6 traffic between neighbors.

Why wrong: This is wrong because EUI-64 is an addressing method, not an encryption feature.