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A wireless access point is a Layer-2-device (like a bridge or a switch) right? Pure Layer-2 switches (without any management capabilitis, etc.) don't have MAC-addresses. They just forward Frames (Layer-2 PDUs) based on the information stored in their bridging tables.

Now when a wireless access point is a bridge between, for example, IEEE 802.3ab (Gigabit Ethernet), and IEEE 802.11n (a variation of Wireless LAN), then why does it have a Layer-2 address assigned?

I heard that the BSSID is the Layer-2 address of the wireless access point. However, if a wireless access point is a Layer-2 device, it shouldn't have a Layer-2 address, right? Just like a switch, for the pure purpose of switching, doesn't have a Layer-2 address.

So what is the BSSID actually?

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2 Answers 2

A wireless access point is a Layer-2-device (like a bridge or a switch) right?

No - That's just simply not how WiFI works. WiFi is NOT Ethernet over Radio.

Pure Layer-2 switches (without any management capabilitis, etc.) don't have MAC-addresses.

Nope - Layer 2 is where MAC lives. You're thinking of Layer 1 (PHY).

They just forward Frames (Layer-2 PDUs) based on the information stored in their bridging tables.

Mostly true...

IEEE 802.11n (a variation of Wireless LAN)

802.11n is a "radio" specification, with a few control extensions, for the base 802.11 protocol.

I heard that the BSSID is the Layer-2 address of the wireless access point.

Correct.

So what is the BSSID actually?

Basic Service Set Identification - In "infrastructure mode" (normal WiFi networks) it's the MAC address of the Access Point. A SSID is the name of the network, the name you type in to identify the network. An ESS identifies multiple APs that share a single SSID, providing roaming, common authentication, etc.

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I know that Layer 2 is "where MAC lives", but you never actually send, say, an Ethernet Frame "to a switch" with the switch as the actual "destination" (unless you think about switches talking to each other for spanning tree and whatnot). Therefore the switch forwards frames for other devices based on Layer 2 information, but it shouldn't require a Layer 2 address for itself, since you never use Layer 2 for actually talking "to the switch", you only use it for talking to another ("higher", e. g. Layer 3) device "via the switch". The Layer 3 device is where the (logical) Layer 2 link "ends". –  no.human.being Feb 23 at 16:17
    
Wireless AP's have their own MAC address. Can't AP's have multiple BSSID's for a given MAC address? The "association" framework Wi-Fi provides is like a thin layer over Layer 2, more or less, is it not? –  ultrasawblade Feb 23 at 16:45
    
@ultrasawblade No. MACs are unique to BSS just as SSID is unique to BSS. Some APs are able to support multiple BSS instances. Note that I'm referring to the wireless MAC, not the wire side. –  quadruplebucky Feb 24 at 0:04
    
@no.human.being An Ethernet switch only snoops on MAC addresses for efficiency. It's not a layer 2 device in the traditional sense of the term unless it also has management functions and communicates with devices on the network. –  Chris S Feb 24 at 0:06
    
@ChrisS An ethernet switch FUNDAMENTALLY operates at layer 2 and relies on MAC addresses to build forwarding tables. Otherwise it would be a hub. –  quadruplebucky Feb 24 at 0:10

With ethernet, the signal broadcast follows a very simple path, the twisted pairs in the cable connected to the ethernet port on the originating device and the switch. Given that a radio broadcasts its signal in a much less controlled manner, how would you propose that Layer 2 deals with who should receive the signal?

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Yeah thanks, that makes sense. So the BSSID is basically (among other things) used to tell the access point that "I, [Source MAC], want to reach [Destination MAC] through [BSSID]", right? –  no.human.being Feb 23 at 16:38
    
Yeah, you should also consider it a centralized controller, ensuring that multiple nodes are not broadcasting at the same time (to avoid collisions) and so forth. –  NickW Feb 24 at 9:09

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