Switching Technologies
Switching technologies and features
What you will learn
- Explain how a switch builds its MAC address table from incoming traffic.
- Distinguish forwarding a frame from flooding it, based on destination knowledge.
- Distinguish the Store-and-Forward and Cut-Through switching methods.
- Explain the broadcast domain concept and its relation to a switch.
When a switch first powers on, it has no idea which device sits on which port — it builds that knowledge gradually from the actual traffic itself, not from any prior configuration.
How a switch learns MAC addresses
1. Receive a frame
Records the incoming frame's source MAC address along with the port it arrived on.
2. Look up the destination
Looks for the destination MAC in its table — if found, sends the frame only to that port (forward).
3. Flood when unknown
If the destination isn't in the table, it sends the frame to every port except the one it arrived on (flood) until it learns from the reply.
Core concepts
| MAC Address Table | A dynamic table mapping each MAC address to the port it was last seen on — built automatically and continuously updated. |
|---|---|
| Broadcast Domain | The set of devices that receive all broadcast traffic — a basic switch doesn't split the broadcast domain; a router (or a VLAN) does. |
| Unknown Unicast Flooding | Sending a frame to every port when its destination isn't known yet — a temporary behavior that stops once the destination is learned. |
Store-and-Forward vs. Cut-Through
| Store-and-Forward | Cut-Through | |
|---|---|---|
| How it works | Receives the whole frame, checks for errors, then sends it | Starts forwarding as soon as it reads the destination address, without waiting for the whole frame |
| Latency | Higher — waits for the full frame | Lower — starts forwarding immediately |
| Error detection | Catches corrupted frames before forwarding them | May pass along a corrupted frame since it never checks it fully |
Common confusion
A basic switch doesn't segment the broadcast domain at all — every device connected to it shares one broadcast domain, unless VLANs are added to split it logically. Actually splitting the broadcast domain is the router's job (or VLANs).
Quick check
Answer in your head first, then reveal.
How does a switch learn a new device's MAC address?Reveal the answer
From the source MAC address in the first frame received from that device, along with the port number.
What does a switch do when it doesn't know the frame's destination port?Reveal the answer
It floods the frame to every port except the one it arrived on.
Which switching method has lower latency?Reveal the answer
Cut-Through, because it starts forwarding as soon as it reads the destination address, without waiting for the whole frame.
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Sources
Used to verify the facts. The writing is original to Passuit.