IPv6 Address Not Showing on Interface After Configuration
Presenting Symptom
After configuring an IPv6 address on a router interface, the 'show ipv6 interface brief' command does not display the address, and the interface shows 'unassigned'.
Network Context
A small branch office network with a Cisco 4321 router running IOS XE 16.9. The router connects to an ISP via GigabitEthernet0/0/0 and to internal switches via GigabitEthernet0/0/1. The engineer configured an IPv6 address on GigabitEthernet0/0/1 but it does not appear in the output.
Diagnostic Steps
Verify IPv6 configuration on the interface
show running-config interface GigabitEthernet0/0/1interface GigabitEthernet0/0/1 ipv6 address 2001:db8:1::1/64 no shutdown
Check if the IPv6 address is present in the running config. If missing, the configuration was not applied correctly. If present, proceed to check interface status.
Check interface status and IPv6-specific state
show ipv6 interface GigabitEthernet0/0/1GigabitEthernet0/0/1 is up, line protocol is up IPv6 is enabled, link-local address is FE80::/64 [TEN] No global unicast address is configured
Look for 'IPv6 is enabled' and the presence of a global unicast address. If 'No global unicast address is configured' appears, the address was not applied. Also check if the interface is administratively down.
Check for IPv6 unicast-routing and interface-level IPv6 enablement
show running-config | include ipv6 unicast-routingipv6 unicast-routing
If 'ipv6 unicast-routing' is missing, IPv6 forwarding is disabled globally. However, this does not prevent address assignment. More importantly, check if the interface has 'ipv6 enable' configured if no address is assigned.
Verify that the interface is not in a shutdown state
show interfaces GigabitEthernet0/0/1GigabitEthernet0/0/1 is up, line protocol is up
If the interface is administratively down, IPv6 addresses will not be assigned. Look for 'administratively down' in the output.
Root Cause
The interface GigabitEthernet0/0/1 is administratively down (shutdown) due to a previous configuration error. When an interface is shut down, IPv6 addresses configured on it are not applied and the interface shows 'unassigned' in 'show ipv6 interface brief'.
Resolution
Verification
Run 'show ipv6 interface brief' and 'show ipv6 interface GigabitEthernet0/0/1'. Expected output: GigabitEthernet0/0/1 [up/up] FE80::/64 2001:DB8:1::1 The interface should show 'up/up' and the global unicast address should be listed.
Prevention
1. Always verify interface status after configuration changes using 'show ip interface brief' or 'show ipv6 interface brief'. 2. Use 'no shutdown' as part of the initial interface configuration template. 3. Implement configuration management and change control to avoid accidental shutdowns.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why an IPv6 address is not showing. The exam tests the understanding that an interface must be in 'up/up' state for IPv6 addresses to be assigned. Candidates should know the 'show ipv6 interface brief' command and the impact of 'shutdown' on address assignment.
Exam Tips
Remember that 'show ipv6 interface brief' shows only interfaces with IPv6 enabled; a shutdown interface will not display its configured address.
The 'ipv6 enable' command is required for link-local address generation if no global address is configured, but a global address can be assigned without it.
Always check interface status first when an IPv6 address is missing; 'shutdown' is a common misconfiguration.
Commands Used in This Scenario
show interfaces
Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.
show ipv6 interface brief
Displays a summary of IPv6 interface status and addresses, useful for quickly verifying IPv6 configuration and interface operational state.
show running-config
Displays the current active configuration in DRAM, showing all non-default settings.
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