Only Link-Local IPv6 Address Present — No Global Unicast
Presenting Symptom
A host on the IPv6-enabled network can only obtain a link-local address (fe80::/10) and does not receive a global unicast address (2000::/3).
Network Context
This issue occurs in a small branch office with a Cisco router (IOS 15.x) acting as the IPv6 default gateway and a DHCPv6 server. The router is configured with SLAAC and DHCPv6 stateless (Router Advertisement with O flag set). The host is a Windows 10 PC connected via Ethernet. The problem manifests after a router configuration change or reboot.
Diagnostic Steps
Check IPv6 address on the host
ipconfig /all (Windows) or ip addr show (Linux)Ethernet adapter Ethernet: Connection-specific IPv6 Address: fe80::...%12 (Link-Local) No other IPv6 addresses listed.
The host only has a link-local address, indicating it did not receive a global unicast address via SLAAC or DHCPv6.
Verify Router Advertisement on the router interface
show ipv6 interface gigabitethernet 0/0/0IPv6 is enabled, link-local address is fe80::... Joined group address(es): FF02::1 FF02::2 FF02::1:FF... 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 (default) ND advertised reachable time is 0 (unspecified) ND advertised retransmit interval is 0 (unspecified) ND router advertisements are sent every 200 seconds ND router advertisements live for 1800 seconds Hosts use stateless autoconfig for addresses.
Check if the interface has a global unicast address configured. If not, the router cannot advertise a prefix. Also verify that 'ipv6 nd other-config-flag' is set if using DHCPv6 stateless.
Check IPv6 routing and prefix delegation
show ipv6 routeIPv6 Routing Table - default - 3 entries Codes: C - Connected, L - Local, S - Static ... C 2001:DB8:1:1::/64 [0/0] via GigabitEthernet0/0/0, directly connected L 2001:DB8:1:1::1/128 [0/0] via GigabitEthernet0/0/0, receive
Ensure the router has a connected global unicast prefix on the host-facing interface. If missing, the router cannot include a prefix in Router Advertisements.
Verify Router Advertisement prefix and flags
debug ipv6 nd (or show ipv6 nd prefix)Router Advertisement from FE80::... on GigabitEthernet0/0/0: ICMP type = 134 (Router Advertisement) ... Prefix 2001:DB8:1:1::/64 onlink autoconfig Valid lifetime 2592000, preferred lifetime 604800 Flags: O (Other config) = 1, M (Managed) = 0
Check that the prefix is included and the A flag (autoconfig) is set for SLAAC. If the O flag is set but DHCPv6 server is unreachable, the host may not get a global address.
Root Cause
The router interface GigabitEthernet0/0/0 does not have a global unicast IPv6 address configured, so it cannot advertise a prefix via Router Advertisements. The host only receives a link-local address because there is no prefix to autoconfigure from.
Resolution
Verification
Run 'show ipv6 interface gigabitethernet 0/0/0' and verify that a global unicast address is listed. Then on the host, run 'ipconfig /all' and confirm a global unicast address appears (e.g., 2001:DB8:1:1::...).
Prevention
1. Always configure a global unicast IPv6 address on router interfaces that should provide SLAAC. 2. Use a consistent prefix delegation from an upstream router or ISP. 3. Verify IPv6 configuration with 'show ipv6 interface brief' after changes.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario tests understanding of IPv6 address autoconfiguration (SLAAC) and the role of Router Advertisements. Questions may present a troubleshooting scenario where a host only has a link-local address, and the candidate must identify that the router lacks a global unicast address or that the prefix is missing from RAs.
Exam Tips
Remember that a router must have a global unicast address on the interface to include a prefix in Router Advertisements.
The 'ipv6 nd prefix' command can override default prefix advertisement; ensure the A flag is set for SLAAC.
Use 'debug ipv6 nd' cautiously in the exam simulation to see RA details, but know the key flags: A (autoconfig), O (other config), M (managed).
Commands Used in This Scenario
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