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In our network, we have several Cisco 1721 routers located at branch offices connected back to the main office via T-1 circuits. All the 1721s are running the same IOS and configured the same, with static global IPv6 addresses on the LAN interface. Here are the relevant configuration items:

ipv6 unicast-routing
interface FastEthernet0
 ip address
 no ip proxy-arp
 speed auto
 ipv6 address 2001:xxxx:xxxx:22::1/64
 ipv6 enable
interface Serial0
 ip address
 no ip proxy-arp
 ipv6 enable
 no ipv6 nd suppress-ra
 priority-group 1
ipv6 route ::/0 Serial0

But I have one router where the FastEthernet0 interface is not initializing IPv6 properly. The IPv6 addresses stay at tentative status:

FastEthernet0 is up, line protocol is up
  IPv6 is enabled, link-local address is FE80::20C:30FF:FE8E:B1F0 [TENTATIVE]
  Global unicast address(es):
    2001:xxxx:xxxx:22::1, subnet is 2001:xxxx:xxxx:22::/64 [TENTATIVE]
  Joined group address(es):
  MTU is 1500 bytes
  ICMP error messages limited to one every 100 milliseconds
  ICMP redirects are enabled
  ND DAD is enabled, number of DAD attempts: 1
  ND reachable time is 30000 milliseconds
  ND advertised reachable time is 0 milliseconds
  ND advertised retransmit interval is 0 milliseconds
  ND router advertisements are sent every 200 seconds
  ND router advertisements live for 1800 seconds
  Hosts use stateless autoconfig for addresses.

What could cause this to happen? Where should I start troubleshooting next?

I did try ipv6 nd dad attempts 0 on that interface with no success.

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1721's went out of software support in 2010 when Cisco's CPE-side IPv6 was still pretty green. Sadly you've likely hit an IOS bug that will never be fixed.

If your configs are identical (except for version) and the IOS versions are identical (checksumming the boot image might not be a bad idea) then if a reboot doesn't help you're probably out of luck.

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