If we were able to clone the MAC address on an access point, what would clients do when faced with two APs with the same MAC and SSID? Will this AP appear as a single device? Will the clients just randomly send information to one AP or the other, or will they flip flop between the two?

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    Clients would get confused. They wouldn't be able to tell one from the other and traffic would randomly go to one or the other based on how the ARP table got updated (different systems do this differently, so not all clients will react the same way). It'd also confuse upstream devices (the router) which would relay traffic back to the wrong AP at times. Don't duplicate MACs on a LAN, ever. – MaQleod Dec 30 '17 at 8:18
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    Possible duplicate of Assigning same MAC Address to access points – MaQleod Dec 30 '17 at 8:19
  • @MaQleod: An AP is a bridge and its MAC address is never used by the router (except when connecting to the AP for management -- but I think OP is asking about changing the Wi-Fi MAC, not the Ethernet one). The router deals directly with clients' MACs. – user1686 Dec 30 '17 at 11:10
  • @grawity, bridges are still layer 2 devices and require MAC addresses in order to talk to other layer 2 devices. That still requires a forwarding table in order to send to the port that the device is plugged into. But you're right - the MAC presented to clients is probably not the same one that is presented to the router (though to be honest I really haven't looked that closely at access points in that regard). – MaQleod Dec 30 '17 at 11:18
  • No, that's exactly why they don't need a MAC address from the Ethernet perspective – they're no different from e.g. an Ethernet switch, which only has a MAC for management but not for regular forwarding. (But again, the thread is about duplicating the WiFi BSSID, which is a different matter.) – user1686 Dec 30 '17 at 13:39

If you set up two devices to act as APs with the same wireless MAC address (BSSID) and same network name ([E]SSID), assuming you don't also load special software on them to allow them to truly work as a single AP as per the IEEE 802.11 specification for what an AP is required to do, then clients that are within radio range of both devices will effectively see a single AP behaving in ways that break the 802.11 standard, and will probably be unable to use join "the AP it sees" (i.e. unable to join either AP), and will be unable to use the network reliably.

If the two APs are on separate, non-overlapping channels, there's a chance it might partially work, because the client might not notice the conflicting channel information in the beacons and probe responses it receives from that BSSID when scanning, so it might arbitrarily use one of the channels. Once it tunes to that one channel and stays on it, it will only be communicating with one of the APs, so things might work okay until it has some reason to go scan, at which point it may get confused by seeing its AP report a different channel than it expected.

If the two APs are on the same channel, the client might be unable to join the network at all, because the authentication and association exchanges won't go well with two different AP devices responding from the same BSSID. If you're using WPA2-PSK security, you won't get past the key handshake because of this. Even if the client did get associated, the competing beacons, with their competing TSF timestamp values, will screw up the timer synchronization functionality of the client's 802.11 chipset, so it seems like things will be unstable at best.

Please note that it IS possible to have multiple APs use the same BSSID and have it work, but it requires special software on the APs to coordinate their actions with each other so they (as a group) never behave in a way that violates the IEEE 802.11 spec for how an AP must behave. At least one vendor of enterprise-class 802.11 APs has done this in the past (I think it was either Meru or Meraki).

  • Thank you for a very clear answer. Would you also be able to explain how some APs can use the same BSSID with special software? What does that involve in the background? I know Unifi APs were able to do it in the past with what they called zero handoff. – ayao1337 Jan 2 '18 at 2:56

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