I am new to networks. What does it mean when a switch doesn't learn the MAC address of the connected device? And how can VLANs can impact that?
Switching – Why a Switch Doesn’t Learn MAC of a Connected Device
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Switches don't use ARP to learn MAC addresses. Switches lean which MAC address is on which port simply by inspecting the source addresses on frames which come into the switch ports.
Switches don't understand layer-3 (except for layer-3 switches). To the switch, a router is just another host on the LAN.
All the above is from the perspective of layer-2 switch operation. A switch may have a management address, but, again, this is just another host on the LAN, and the management will use ARP and have a configured gateway and learn its MAC address with ARP, but it really has nothing to do with how the switching operates.
A host wishing to send an IP packet to another host will look in its ARP cache for the MAC address. If there isn't an entry in its ARP cache, it will send an ARP request to resolve the layer-3 IP address to the layer-2 MAC address. It will then build a layer-2 frame with this information and send it to the switch. If the destination IP address is on a different network, the host will look for the MAC address of its configured gateway in its ARP cache, and send an ARP request if it isn't in there, build a frame and send it out, just as it would for any other host.
As the frames come into the switch, the switch will look at the source MAC addresses, and use that to build its MAC address table, not the same as an ARP cache. When a switch receives a frame with a destination MAC address which isn't in its MAC address table, it will flood the frame out every port; it does not use ARP requests to discover this. It doesn't take a switch very long to build its MAC table since it take only one frame from each host to populate the MAC address table.
Good question. I'll answer it with an animation:
When Host A sends the frame, the switch does not have anything in its MAC address table. Upon receiving the frame, it records Host A's MAC Address to Switch Port mapping. Since it doesn't know where the destination MAC address is, it floods the frame out all ports.
This assures that if host B exists (which at this point, the switch does not know yet), that it will receive it. Hopefully, upon receiving the frame, Host B will generate a response frame, which will allow the Switch to learn the MAC address mapping from the return frame.
You can read more about how a Switch works here (where I took the animation from). I would also suggest reading the entire article series for a closer look at how a packet moves through a network.
One last note regarding the terms Flooding vs Broadcast. A switch never broadcasts frames, a broadcast is not an action a switch can take. A switch can only flood a frame. A broadcast is simply a frame with a destination MAC address of ffff.ffff.ffff. This is often confused because the end effect is the same, but they are actually different.
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A switch has a MAC address table for each VLAN, and the switch will populate the MAC address table of the VLAN with the source MAC address of any frame entering the switch on an interface in that VLAN.
You did not specify the switch model, so we cannot tell you specifically how to see the MAC address table. On a Cisco switch, the
show mac-address-tablecommand will show all the entries for all the MAC address tables, but other vendors may do it differently.Also, MAC address table entries will time out, so if the switch has not seen any frames from a particular host in a while, the MAC address table entry for that host will time out of the table.
In your comment, you explain that you have another bridge (switches and WAPs are bridges). If a host on your WAP sends frames to another host on the WAP, the switch will not see those frames, so it will not have an entry in its MAC address table for those hosts. It is only when a host on the WAP sends a frame that enters the switch that the switch will create an entry for the MAC address of the host. The switch interface for the WAP could have multiple MAC addresses associated with it in the switch MAC address table if multiple hosts on the WAP send frames into the switch, but the key is that the switch must see at least one frame for a host enter the switch during its timeout period for you to see the MAC address in its MAC address table.
The bridges will not normally have MAC address table entries for other bridges because the bridges do not originate traffic, so the frames will not have the source MAC address associated with the bridge. Bridges normally forward frames transparently. A WAP must translate the Wi-Fi frames into ethernet frames, and vice versa, but it preserves the original host source and destination MAC addresses of the original frames on the translated frames.