Neighbours#
The second view on Interfaces, reached from the segmented control in the header. It is the router's cache of addresses it has resolved to hardware addresses, for IPv4, in a table.
It is read-only. There is no editor for a static ARP entry here, and no button to flush the cache. The agent caches the read for a few seconds, so two open tabs do not double the load on the router.
The columns#
| Column | What it is |
|---|---|
| Address | the address that was resolved |
| Interface | the interface the answer came in on |
| MAC address | the hardware address that answered, or — when there is none yet |
| State | the neighbour state for the entry |
Reading the State column#
The badge is green for Reachable and Permanent, red for Failed and Incomplete, and grey for everything else.
| State | What it means in practice |
|---|---|
| Reachable | confirmed recently; traffic to this address is going somewhere real |
| Stale | it answered once and has not been confirmed since. Normal for a quiet host |
| Delay, Probe | the kernel is in the middle of re-confirming it |
| Incomplete | a request went out and nothing has answered yet |
| Failed | nothing answered. The address is configured somewhere but nothing on that segment holds it |
| Permanent | a static entry, added by hand |
A Failed entry for a gateway address is the fastest confirmation that an uplink is dead at layer 2 rather than upstream. A Failed entry for a host you expected to be present usually means a VLAN mistake: the address is right, the segment is wrong.
An empty table is not an error. The cache fills as devices exchange traffic with the router.
What this table cannot tell you#
- It is a cache, not an inventory. A host that has not spoken recently is not in it. Absence proves nothing.
- It has no history. There is no record of which hardware address held an address yesterday, so it will not help you find a duplicate that has already resolved itself.
- It is IPv4. IPv6 neighbours are on IPv6.
- There is no flush and no static entry.
The other neighbour view#
The same data appears on the Diagnostics page beside LLDP — what the switches on the other end of each cable announce about themselves. For which switch port is this, use that view; for is this host alive on this segment, this one is enough. See Diagnostics → Neighbours.
See also#
- Interfaces — the page this view belongs to.
- Diagnostics → Neighbours — the same table beside LLDP.
- Static routes — when the neighbour is a gateway, this is where its route lives.
- DHCP — the lease table, which is the other list of who is on the segment, and the one with hostnames in it.