Viewing the Neighbor Cache

Neighbor discovery occurs when there is communication between IPv6 devices on a VLAN. The Neighbor Cache retains data for a given neighbor until the entry times out. For more on this topic, see Neighbor Discovery (ND).


show ipv6 neighbors [vlan <vid>]

Displays IPv6 neighbor information currently held in the neighbor cache. After a period without communication with a given neighbor, the switch drops that neighbor's data from the cache. The command lists neighbors for all VLAN interfaces on the switch or for only the specified VLAN. The following fields are included for each entry in the cache:

IPv6 Address: Lists the 128-bit addresses for the local host and any neighbors (on the same VLAN) with whom there has been recent communication.

MAC Address: The MAC Address corresponding to each of the listed IPv6 addresses.

VLAN <vid> : Optional. Causes the switch to list only the IPv6 neighbors on a specific VLAN configured on the switch.

Type: Appears only when VLAN is not specified and indicates whether the corresponding address is local (configured on the switch) or dynamic (configured on a neighbor device).

Age: Appears only when the VLAN is specified and indicates the length of time the entry has remained unused.

Port: Identifies the switch port on which the entry was learned. If this field is empty for a given address, the address is configured on the switch itself.

State: A neighbor destination is reachable from a given source address if confirmation has been received at the source verifying that traffic has been received at the destination. This field shows the reachability status of each listed address:

INCMP: (Incomplete): Neighbor address resolution is in progress, but has not yet been determined.

REACH: (Reachable): The neighbor is known to have been reachable recently.

STALE: A timeout has occurred for reachability of the neighbor, and an unsolicited discovery packet has been received from the neighbor address. If the path to the neighbor is then used successfully, this state is restored to REACH.

DELAY: Indicates waiting for a response to traffic sent recently to the neighbor address. The time for determining the neighbor's reachability has been extended.

PROBE: The neighbor may not be reachable. Periodic, unicast neighbor solicitations are being sent to verify reachability.

The output for show ipv6 neighbors command is:
Switch(config)# show ipv6 neighbors
 IPv6 ND Cache Entries
 IPv6 Address                MAC Address   State Type    Port
 ––––––––––––––––––––––––––– ––––––––––––– ––––– ––––––– ––––
 2001:db8:260:212::101       0013c4–dd14b0 STALE dynamic 1
 2001:db8:260:214::1:15      001279–88a100 REACH local
 fe80::1:1                   001279–88a100 REACH local
 fe80::10:27                 001560–7aadc0 REACH dynamic 3
 fe80::213:c4ff:fedd:14b0    0013c4–dd14b0 REACH dynamic 1
The output for neighbor cache content for a specific VLAN is:
Switch(config)# show ipv6 neighbor vlan 10

 IPv6 ND Cache Entries

 IPv6 Address             MAC Address   State Age           Port
 –––––––––––––––––––––––– ––––––––––––– ––––– ––––––––––––– ––––
 2001:db8:260:212::101    0013c4–dd14b0 STALE 5h:13m:44s    1
 2001:db8:260:214::1:15   001279–88a100 REACH 11h:15m:23s   17
 fe80:1a3::1:1            001279–88a100 REACH 9h:35m:11s    12
 fe80:::10:27             001560–7aadc0 REACH 22h:26m:12s   3
 fe80::213:c4ff:fedd:14b0 0013c4–dd14b0 REACH 23 0h:32m:36s 1