What exactly does a switch do with the frames it recieves?

I've become ultra confused all of a sudden.
Switch is a layer 2 device. So this means it deals with frames (builds and examines them). So when it receives a frame, it looks inside to see where the destination mac address is. But since the mac address is on some further away network, and not directly connected to the switch, how in the world can it decide which way to send it? The switch doesn't have a routing table so theres no way of determining. So it either sends it down a trunk link or out all the vlans associated with the source port. Now if its sending the frame out all the vlan ports then this certainly does not break up broadcast or collision domains. Switches have no idea of the topology of a network and therefor must be sending frames out every possible hole they can to get rid of it.
But this would end up being huge problems. If I have a router on a stick topology, with trunk link. And a frame gets sent out all possible ports and a trunk link, the packet is going to get to the source, then after being routed through the router and come back it will be sent to the source again. Also isn't it making a storm if the router routes the packet back to the switch and the switch then sends the same packet down the trunk link, since everything gets sent to the trunk by default?
Or not even with a trunk link. Just linked up to any router. If the mac address isn't directly connected to the switch it must send the packet out every port it can because it doesn't have any idea what else to do with. And if it sends it to the router, and the router says, no it has to go back towards you, wouldn't the packet just go in circles for years with the router saying, "its in your direction", and the switching saying "i don't know where this is supposed to go, so i'll send it anywhere i can"
thanks for any help at all. really frustrated.

Hi There
In your topology example "Host A -> Switch1 -> Switch2 -> Switch3 -> Host B", for Host A to have any chance of communicating with Host B, they would have to be in the same VLAN/Subnet, as you have no layer 3 device to route between different VLAN's/Subnets.
Therefore when Host A wants to send a frame to Host B it would check its own MAC address table to see if it has an entry for Host B. If it does, it will send the packet with the frame towards Host B. The source MAC addresses will be that of Host A and the destination MAC addresses will be that of Host B.
If Host A does not have the MAC address of Host B it will send an ARP request out on to the wire to Sw1.
Either way, as this is a single VLAN/Subnet, the way Sw1 and the other switches treat the frame will be exactly the same.
Sw1 will receive the frame and check the source MAC address against its CAM table. If no entry exists, it will add the MAC of Host A plus the port to which Host A is connected to its CAM table. If an entry does exist, it will then examine the destination MAC address to see if it has an entry for it. If it has an entry it will forward (switch) the frame out through the interface through which Host B can be reached (This would be the uplink port to Sw2 in this case). If no entry exists, it will flood the frame out of all ports which are in the same VLAN as Host A, except the port the frame arrived on.
SW2 will receive this frame and it will check its own CAM table. If it no entry exists for Host A's MAC address, an entry will be added. If an entry does exist, switch be will then check the destination MAC address.
If it has an entry for Host B, it will forward (switch) the frame out of the port through which Host B can be reached. If no entry exists, it will flood the frame out of all ports which are assigned to the same VLAN as Host A. Sw3 will receive this frame.
It will check its CAM table to see if it has an Entry for Host A. If not it will add one. If it has it will check the CAM table for an entry for the MAC address of Host B. If it has one, it will forward (switch) the frame out the port through which Host B can be reached. If no entry is found then Sw2 will Flood the Frame out of all ports which are assigned to the same VLAN as Host A.
Once Host B responds to the initial frame,
Host A will have the MAC of Host B
Sw1 will know that to get to Host B it sends frames out through the port through which it connects to SW2. To get to Host A it sends the frames out through the port to which Host A is connected.
Sw2 will know that to get to Host A it sends frames out through the port to which it connects to SW1. To get to Host B, it sends frames out through the port through which it connects to SW3.
SW3 will know that to get to Host A, it sends frames out through the port through which it connects to SW2. To get to Host B it sends frames out through the port to which Host B is connected.
At all times in this scenario, when Host A sends traffic to Host B, the Frame will have the source MAC of Host A and the destination MAC of Host B. Like wise when Host B sends data to Host A, the frames will have the source MAC of Host B and the destination MAC of Host A.
What you want to understand is how a switch builds its MAC address (CAM) table.
It checks the source MAC first, If an entry exists, then it checks the destination MAC. If no entry exists, then the switch creates and entry and then checks the destination MAC address. If an entry exists the frame will be switched to the relevant port. If no entry exists, then the frame is flooded out of all ports (in the same VLAN as the frame of the originating host) except the port the frame arrived on.
Best Regards,
Michael

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