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Locating devices

A switch learns the MAC address of every device whose traffic passes it, together with the port it came in on. In a network of several switches, a device therefore appears on its own port of the switch it is plugged into, and on the uplink port of every switch between that one and the others.

helpers.locate_macs() takes the reads of all your switches and returns, for every device, the one port that is not an uplink:

from aiosnmpswitch.helpers import SwitchData, locate_macs

switches = {
    "hallway": SwitchData(interfaces=..., bridge_ports=..., fdb=..., lldp=..., lag_ports=...),
    "living-room": SwitchData(...),
}
for mac, where in locate_macs(switches).items():
    print(mac, where.switch, where.port_name, where.devices_on_port, where.ip_addresses)

How it decides:

  1. Uplinks are ports on which LLDP sees another of the given switches, recognised by its chassis ID or one of its interface MAC addresses. A switch that is not in the mapping (an unmanaged switch, a switch without SNMP) counts as a device: everything behind it is located on the port it hangs on.
  2. Address-table entries on uplinks are ignored, as are the switches' own addresses and entries that are not learned (the switch itself, management).
  3. Trunk members are located on their trunk.
  4. If a device is still seen on more than one port, the port with the fewest devices wins.
  5. A device seen only on uplinks (its entry aged out on the edge switch) is left out rather than placed wrongly.

devices_on_port greater than one means something is between the port and the device: an access point with its Wi-Fi clients, a router, a hub or an unmanaged switch. ip_addresses come from the switches' own neighbour caches, which usually know only a few devices; take addresses from your router or DHCP server instead.

Limits

  • LLDP is needed on every switch to recognise the links between them. The link's port is found by the port name the switch sends in LLDP, or, if it sends none, by taking LLDP's port number as the interface index. Switches whose LLDP port numbers are not their interface indexes and whose port IDs match no port name (some stacked switches) show no uplinks; their neighbours' devices then look connected to them.
  • A switch is recognised as one of yours only by a MAC address in its LLDP chassis ID; a chassis ID of another type (a name, an IP address) makes it count as a device.
  • A device that moved from one switch to another is seen on both until the old entry ages out, and a host with two links (a bond, a hypervisor) on two ports: the port with fewer devices wins.
  • A device whose MAC address equals one of a switch's own (a switch that is also your router) is left out.

Address tables age out after about five minutes of silence (the switch's setting), so a device that sends nothing for a while disappears until it talks again.