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Is there any way to detect the underlying layout physical layout of a VLAN. For instance, to know whether two devices in the same VLAN are connected via the same wifi AP.

To clarify, this would be from the perspective of a device connected to the WLC, with no special privileges. If you can operate in monitor mode then of course the underlying structure can be seen from beacons/probes etc, but assuming these are not available to you, as on many devices they aren't, I was wondering if anything could be done 'actively'.

The ping time and % packet loss is an obvious option, but I think there's too much variance in both to give a reliable answer. I suspect the answer is no, unless you have admin access to the switch via SNMP. I just want to check there isn't something clever that could be done with LLDP, DHCP or ARP etc.

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    From what perspective are you mapping this? Logged into an ethernet switch attached to the WLC, logged into the WLC, or as a passive listener to radio waves?
    – Mike Pennington
    Sep 16, 2013 at 23:08
  • I've added to the question to clear this up.
    – infintejestr
    Sep 17, 2013 at 0:09
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin ♦
    Jan 3, 2021 at 20:02

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Without some level of administrative privilege, the short answer is that this is not possible with any degree of accuracy.

As you pointed out, using ping time and or packet loss is dubious at best. You cannot account for where the latency/loss is being introduced, so it is at best a very shaky guess without having some knowledge of the network architecture.

Everything else that has been suggested, looking at MAC forwarding tables, wireless associations, etc. all require privileges, i.e. administrative control of the device, and some of them are still subject to pitfalls (AP to controller tunneling, unmanaged switches/hubs, etc.) that require you to have additional information about the network to reliably interpret you results.

Layer 2 discovery protocols (LLDP, CDP, etc.) for the most part only broadcast information about themselves to the network, so you would still need some level of administrative control to be able to see the received LLDP information at a given point in the network.

I think you best bet of walking into a completely uncontrolled environment and determining this would be sniffing the wireless communications, but you have ruled that out as well.

So no, without some special privileges, this is not possible.

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Assuming you have the two devices's MAC addresses, you could use MAC tables to see on which interface the MACs came in on. If they come in on the same interface then they're associated to the same AP.

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    Same switch interface in MAC table doesn't necessarily mean the devices were associated to the same AP. The WLC is typically trunked and frames for WLC clients only have VLAN tagging and source MACs belong to the clients even when clients are on different APs.
    – generalnetworkerror
    Sep 17, 2013 at 7:48
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to know whether two devices in the same VLAN are connected via the same wifi AP.

If this is a cisco WLC you can view the current associations on any specific AP via a filter to find your two users by mac address. This is found under the "Monitor" tab.

Otherwise Christian Delapena's answer is adequate if you know which port the AP is plugged into. On the other hand this won't work for all AP/WLC configurations since some AP's tunnel traffic back to the controller in a packet. Cisco, for example, uses capwap.

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