📘 CCNA 200-301 v1.1
1.13 Describe switching concepts
1.13.b Frame Switching
What is Frame Switching?
Frame switching is the process that a switch uses to receive, process, and forward Ethernet frames from one port to another.
It happens at Layer 2 (Data Link Layer) of the OSI model.
When a switch receives a frame, it looks at the destination MAC address inside that frame. Based on that MAC address, the switch decides which port to forward the frame to.
Why is Frame Switching Important?
Frame switching is the core function of a network switch.
It allows devices within the same LAN (Local Area Network) to communicate efficiently without unnecessary traffic flooding all ports.
By using MAC address learning and switching logic, the switch makes the network faster, more secure, and more efficient.
How Frame Switching Works – Step by Step
Let’s go through the switching process step by step to understand how frames are handled:
Step 1: Frame Reception
When a device (like a PC) sends an Ethernet frame, it arrives on one of the switch’s interfaces (ports).
The switch receives the frame into its input buffer.
Step 2: MAC Address Learning
The switch reads the source MAC address from the incoming frame.
Then it adds an entry to its MAC address table (CAM table) that looks like this:
| MAC Address | Port | VLAN |
|---|---|---|
| 00A1.B2C3.D4E5 | Fa0/1 | 10 |
This means:
“If you want to send traffic to MAC address 00A1.B2C3.D4E5, forward it out of interface Fa0/1 on VLAN 10.”
Step 3: Frame Forwarding Decision
Next, the switch checks the destination MAC address inside the frame.
- If the MAC address is found in the MAC address table → the switch forwards the frame out the specific port associated with that address.
- If the MAC address is not found (unknown destination) → the switch floods the frame out all ports in the same VLAN, except the one it was received on.
Step 4: Frame Switching
Once the switch knows which port to send the frame to, it performs the actual frame switching — this means moving the frame from the input interface to the output interface internally, without modifying the frame contents (except maybe the frame check sequence if needed).
Step 5: Frame Transmission
Finally, the switch sends the frame out of the correct port toward the destination device.
If the destination device replies, the switch learns that device’s MAC address too, continuing to build its MAC address table.
Types of Frame Switching Methods
There are three main switching methods used by Ethernet switches. Each has different speed and error-handling characteristics.
1. Store-and-Forward Switching
- The switch receives the entire frame before forwarding it.
- It checks the Frame Check Sequence (FCS) at the end of the frame to ensure there are no errors.
- If the frame is valid → it’s forwarded.
- If the frame has errors → it’s dropped.
Advantages:
- Reliable (no error frames forwarded)
- Supports different frame sizes and speeds
Disadvantages:
- Slightly slower, because the switch waits for the entire frame before forwarding.
Used in: Most modern Cisco switches (default method)
2. Cut-Through Switching
- The switch starts forwarding the frame as soon as it reads the destination MAC address, without waiting for the whole frame to arrive.
- It does not check for errors (no FCS verification).
Advantages:
- Very fast — minimal delay.
Disadvantages:
- Can forward corrupted frames (no error check).
Used in: Environments where speed is more important than error detection (e.g., low-latency LANs).
3. Fragment-Free Switching
- A compromise between store-and-forward and cut-through.
- The switch reads the first 64 bytes of the frame before forwarding.
- This helps avoid forwarding most collision fragments, since errors typically occur within the first 64 bytes.
Advantages:
- Faster than store-and-forward.
- More reliable than cut-through.
Disadvantages:
- Slightly less efficient than full cut-through.
Used in: Some performance-optimized networks.
Switching Table (CAM Table)
The Content Addressable Memory (CAM) table is where the switch stores MAC addresses, their associated ports, and VLAN IDs.
Each entry has:
- MAC address
- Port number
- VLAN ID
- Age timer
The age timer removes old entries if no frames are seen from that MAC after a certain time (default: 300 seconds).
This ensures that the switch’s table stays up to date and does not waste memory.
Broadcast, Multicast, and Unknown Unicast Frames
Switches treat different types of destination MAC addresses differently:
| Frame Type | Example MAC Address | Switch Behavior |
|---|---|---|
| Unicast | 00A1.B2C3.D4E5 | Forward to one port (if known) |
| Broadcast | FF:FF:FF:FF:FF:FF | Flood to all ports in VLAN |
| Multicast | Starts with 01:00:5E | Flood or selectively forward depending on configuration |
| Unknown Unicast | Not in MAC table | Flood to all ports in VLAN (except incoming port) |
Summary Table
| Switching Method | Error Checking | Speed | Common Use |
|---|---|---|---|
| Store-and-Forward | ✅ Yes | Slower | Most networks |
| Cut-Through | ❌ No | Fastest | Low-latency networks |
| Fragment-Free | Partial | Medium | Performance-tuned networks |
Key Points to Remember for the CCNA Exam
- Frame switching happens at Layer 2.
- Switches use MAC address tables (CAM tables) to make forwarding decisions.
- The source MAC address is learned when a frame enters a switch port.
- The destination MAC address is used to determine where to forward the frame.
- If unknown, the frame is flooded.
- Three switching methods exist: store-and-forward, cut-through, and fragment-free.
- Most Cisco switches use store-and-forward as the default method.
✅ In Short
Frame switching is how a switch decides where to send Ethernet frames based on MAC addresses.
It’s the fundamental operation that allows efficient, intelligent communication inside a LAN.
