show ipv6 neighbors
Displays the IPv6 neighbor discovery cache, showing the mapping of IPv6 addresses to MAC addresses on directly connected links, used to verify neighbor reachability and troubleshoot IPv6 connectivity.
show ipv6 neighborsWhen to Use This Command
- Verify that a neighboring device's IPv6 address has been resolved to its MAC address after pinging the neighbor.
- Troubleshoot IPv6 connectivity issues by checking if the neighbor's MAC address is present and its state (REACH, STALE, DELAY, PROBE).
- Monitor the IPv6 neighbor table for potential spoofing or duplicate address detection issues.
- Confirm that a static IPv6 neighbor entry has been configured correctly.
Command Examples
Basic show ipv6 neighbors output
show ipv6 neighborsIPv6 Address Age Link-layer Addr State Interface 2001:db8:acad:1::1 0 aabb.cc00.0100 REACH GigabitEthernet0/0 2001:db8:acad:1::2 12 aabb.cc00.0200 STALE GigabitEthernet0/0 fe80::1 0 aabb.cc00.0100 REACH GigabitEthernet0/0 fe80::2 30 aabb.cc00.0200 DELAY GigabitEthernet0/0
IPv6 Address: The IPv6 address of the neighbor. Age: Number of minutes since the entry was last refreshed (0 means just updated). Link-layer Addr: The MAC address of the neighbor. State: Reachability state (REACH = recently confirmed reachable, STALE = not confirmed in a while, DELAY = waiting for confirmation, PROBE = actively sending NS). Interface: The local interface through which the neighbor is reachable.
Show IPv6 neighbors on a specific interface
show ipv6 neighbors interface gigabitethernet 0/1IPv6 Address Age Link-layer Addr State Interface 2001:db8:acad:2::1 5 aabb.cc00.0300 REACH GigabitEthernet0/1 2001:db8:acad:2::2 45 aabb.cc00.0400 STALE GigabitEthernet0/1 fe80::3 0 aabb.cc00.0300 REACH GigabitEthernet0/1
This filters the output to show only neighbors on GigabitEthernet0/1. The fields are the same as the basic output. Note that link-local addresses (fe80::) are also displayed. A STALE state indicates the entry has not been refreshed recently; the router will attempt to verify reachability when sending traffic.
Understanding the Output
The 'show ipv6 neighbors' command displays the IPv6 neighbor discovery cache. Each row represents a neighbor that the router has learned about through Neighbor Solicitation (NS) and Neighbor Advertisement (NA) messages. The 'IPv6 Address' column shows the neighbor's IPv6 address, which can be global unicast, unique local, or link-local. The 'Age' column indicates how many minutes have passed since the entry was last updated; a value of 0 means the entry was just learned or refreshed. The 'Link-layer Addr' is the MAC address of the neighbor. The 'State' column is critical for troubleshooting: 'REACH' means the neighbor is reachable and recently confirmed; 'STALE' means reachability has not been confirmed recently but the entry is still valid; 'DELAY' means the router is waiting for a confirmation after sending a unicast NS; 'PROBE' means the router is actively sending NS messages to verify reachability. 'INCOMPLETE' indicates that the router has not received a response to an NS. The 'Interface' shows which local interface connects to that neighbor. In a healthy network, most entries should be in REACH or STALE state. A large number of INCOMPLETE entries may indicate connectivity issues or misconfigured devices. Watch for duplicate MAC addresses or unexpected entries that could indicate spoofing.
CCNA Exam Tips
CCNA exam tip: The neighbor state 'INCOMPLETE' means the router sent an NS but hasn't received an NA yet; this is common when pinging a new address for the first time.
CCNA exam tip: Remember that link-local addresses (fe80::/10) are also displayed; they are automatically generated and used for neighbor discovery and routing protocols.
CCNA exam tip: The 'Age' field resets to 0 when the entry is refreshed; a high age with STALE state is normal for idle neighbors.
CCNA exam tip: Static IPv6 neighbor entries can be configured with 'ipv6 neighbor' and will show state 'REACH' with age '-'; they are used for security or to map addresses without ND.
Common Mistakes
Mistake 1: Confusing 'show ipv6 neighbors' with 'show ip neighbors' (which is for IPv4 ARP). The IPv6 command is specifically for IPv6 ND cache.
Mistake 2: Assuming a STALE state indicates a problem; STALE is normal for entries not recently used. The router will verify reachability when needed.
Mistake 3: Forgetting that the output includes link-local addresses; these are essential for IPv6 operations but can clutter the output if not filtered.
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