You have an IPv6 subnet that is on a switch connected to a Cisco router. The clients on the subnets attached to the router aren't getting IPv6 addresses from auto config. What is the most likely cause of this issue?

You have an IPv6 subnet that is on a switch connected to a Cisco router. The clients on the subnets attached to the router aren't getting IPv6 addresses from auto config. What is the most likely cause of this issue?

• DHCP relay is not working
• DHCP is not working
• The DNS server is down
• Missing "ipv6 unicast routing" command 

EXPLANATION

Cisco routers do not have IPv6 routing enabled by default. To configure IPv6 on a Cisco routers, you need to do two things:

1. enable IPv6 routing on a Cisco router using the ipv6 unicast-routing global configuration command. This command globally enables IPv6 and must be the first command executed on the router.
2. configure the IPv6 global unicast address on an interface using the ipv6 address address/prefix-length [eui-64] command. If you omit omit the eui-64 parameter, you will need to configure the entire address manually. After you enter this command, the link local address will be automatically derived.

Here is an IPv6 configuration example:

R1(config)#ipv6 unicast-routing
R1(config)#int Gi0/0
R1(config-if)#ipv6 address 2001:0BB9:AABB:1234::/64 eui-64

We can verify that the IPv6 address has been configured by using the show ipv6 interface Gi0/0 command:

R1#show ipv6 interface Gi0/0
GigabitEthernet0/0 is up, line protocol is up
  IPv6 is enabled, link-local address is FE80::201:42FF:FE65:3E01
  No Virtual link-local address(es):
  Global unicast address(es):
    2001:BB9:AABB:1234:201:42FF:FE65:3E01, subnet is 2001:BB9:AABB:1234::/64 [EUI]
  Joined group address(es):
    FF02::1
    FF02::2
    FF02::1:FF65:3E01
  MTU is 1500 bytes
  ....