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Our network switches have STP enabled, and I see that my APs have an option to enable STP as well. The purpose of STP is to prevent network path loops, but the AP only has one connection back to the switch, and only one connection to each client. Why would I want to enable STP on an AP, or would enabling it be moot?

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If the APs support bridging or routing (especially with vLANs) I could see a use; otherwise you're right, useless. – Chris S Apr 26 '12 at 17:02
Yeah - I can imagine use cases for spanning tree if you've got a mesh network of access points, or you're using a switch built into the AP as an actual switch - but in normal deployments, it's not useful. – Shane Madden Apr 26 '12 at 17:09

In most deployments it is not necessary to configure STP on an access point. However, there are several deployment scenarios where it is advisable.

One such scenario I encountered in production when it was biting the network in the proverbial posterior.

Instead of running a physical cable to connect a printer on a shop floor, an access point radio was configured in what is called "workgroup bridge" role. In this case the radio interface plays the role of uplink to the LAN (associating to another access point that is actually plugged into the LAN) while the physical Ethernet interface connects to the printer. The printer worked great for a year until the business needs changed.

At some point additional printers were needed and a few physical cable runs and a switch (as part of an intermediate distribution frame) were installed at the shop floor location. The printers were plugged into the new switch -- which was plugged/uplinked into the LAN from the new runs. The access point physical Ethernet interface that was once plugged into a printer was also plugged into the new switch.

With both the access point physical Ethernet interface and the radio interface connected to the same broadcast domains -- a loop was formed. Storm-control was in place and caught the broadcast storm in both places. However, if it was not in place that would have been a very tricky loop indeed.

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What if someone configures a wireless bridge device to connect to the AP, then connects the other end of it to a physical switch?

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Nothing will happen unless the configuration of the access point is changed (for example, enabling WDS). The access point will only talk to its clients, not devices they bridge. – David Schwartz May 11 '12 at 1:13
So you're saying if I have a wireless bridge (acting as a client) connected to another access point, and then I connect them both via ethernet also, I'm not going to get a routing loop? It seems that to me any packets the AP receives over wired, it's going to transmit, and any packets the bridge receives over wireless, it's going to send via ethernet. – devicenull May 11 '12 at 1:50
No, you won't. Since the wireless bridge is acting as a client, the access point will not send it packets not bound to its WiFi hardware address, nor will it accept from it packets not from its WiFi hardware address. The WiFi protocol was designed this way back when it topped out at 11Mbps -- bridging by default to 100Mbps networks would have been bad. – David Schwartz May 11 '12 at 18:12
Even in the case of broadcasts? Lets say a broadcast comes from the ethernet to the AP. The AP forwards it to the bridge, because it's a broadcast packet. The bridge sees it and forwards it to the wired network, where the AP sees it, and forwards it again. This seems to have very little to do with the fact that it's wifi. – devicenull May 11 '12 at 20:42
That can't happen. A WiFi bridge will not make itself a client to an access point because the bridge (unless it's fundamentally broken) knows that unicast won't work in that configuration. Broadcast loops would be the least of your problems, and WiFi bridges are smart enough not to set up configurations that can't possibly work at all. – David Schwartz May 11 '12 at 21:09

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