Switch
Summary Table of Switch Methods
| Method | How It Works | Speed | Error Checking | Frame Processing Start | Notes / Use Cases |
|---|---|---|---|---|---|
| Store-and-Forward | Receives entire frame, checks for errors (CRC), then forwards if valid. | Slowest | Best (CRC check) | After full frame received | High accuracy, supports different port speeds |
| Cut-Through | Starts forwarding after reading destination MAC (first 6–8 bytes), before whole frame arrives. | Fastest | None | After 1st 6–8 bytes | Lowest latency, may forward corrupted frames |
| - Fragment-Free | Waits for first 64 bytes (to filter out runts), then forwards. | Moderate | Limited (runts) | After 64 bytes received | Balances speed and error avoidance (filters runts) |
| - Fast-Forward | Often used as another term for cut-through (vendor-specific); behaves like cut-through. | Fastest | None | After 1st 6–8 bytes | Sometimes used interchangeably with cut-through |
Method Details
Store-and-Forward
- Switch receives the entire Ethernet frame into memory.
- Performs a CRC (Cyclic Redundancy Check) to detect errors.
- Only forwards error-free frames.
- Introduces the most latency but ensures data integrity.
- Can connect ports of different speeds.
Cut-Through
- Switch reads only enough of the frame to learn the destination MAC (typically first 6–8 bytes).
- Starts forwarding immediately after determining the output port.
- Lowest latency, but may forward frames with errors.
- Cannot detect or filter out corrupted frames.
- Fragment-Free
- Switch waits for the first 64 bytes of the frame before forwarding.
- 64 bytes is the minimum legal Ethernet frame size; anything smaller is a "runt" (likely a collision fragment).
- Filters out runts but does not check for other errors.
- Provides a balance between speed and error avoidance.
- Fast-Forward
- Often a vendor-specific term for cut-through switching.
- Behavior is essentially the same as cut-through: forwards frames as soon as the destination MAC is read.
- Focuses on minimizing latency.
Visual Comparison
| Feature | Store-and-Forward | Cut-Through | Fragment-Free | Fast-Forward |
|---|---|---|---|---|
| Latency | High | Low | Medium | Low |
| Error Detection (CRC) | Yes | No | No (except runts) | No |
| Filters Runts | Yes | No | Yes | No |
| When Forwarding Starts | After full frame | After 6–8 bytes | After 64 bytes | After 6–8 bytes |
| Port Speed Conversion | Yes | No | No | No |
In summary:
- Store-and-forward maximizes reliability but adds delay.
- Cut-through (and fast-forward) minimizes delay but may propagate errors.
- Fragment-free offers a compromise, filtering out the most common collision fragments but not all errors.
Ethernet Frame Fields
An Ethernet frame is a structured packet used for data transmission at the data link layer. It encapsulates both addressing and error-checking information, along with the payload. Here are the main fields in a standard Ethernet II frame:
Ethernet Frame Structure
| Field | Size (Bytes) | Description |
|---|---|---|
| Preamble | 7 | Synchronizes the receiver's clock; pattern of alternating 1s and 0s |
| Start Frame Delimiter | 1 | Marks the end of preamble and start of the frame (10101011) |
| Destination MAC Address | 6 | MAC address of the receiving device |
| Source MAC Address | 6 | MAC address of the sending device |
| EtherType/Length | 2 | Indicates protocol type (e.g., IPv4, IPv6) or payload length |
| (Optional) VLAN Tag | 4 | Used for VLAN identification (802.1Q) |
| Payload (Data and Pad) | 46–1500 | Data being transmitted; padded if less than 46 bytes |
| Frame Check Sequence | 4 | CRC for error checking |
Field Descriptions
- Preamble: 7 bytes of alternating 1s and 0s for synchronization.
- Start Frame Delimiter (SFD): 1 byte (10101011) indicating the start of the actual frame.
- Destination MAC Address: 6 bytes specifying the intended recipient.
- Source MAC Address: 6 bytes specifying the sender.
- EtherType/Length: 2 bytes; EtherType (≥1536) specifies the upper-layer protocol, or Length (≤1500) for payload size.
- VLAN Tag (optional): 4 bytes inserted after the source MAC for VLAN tagging.
- Payload (Data and Pad): Actual data; minimum 46 bytes (padded if needed), maximum 1500 bytes.
- Frame Check Sequence (FCS): 4 bytes CRC for error detection.
Visual Representation
| Preamble | SFD | Dest MAC | Src MAC | EtherType/Length | [VLAN Tag] | Payload (Data+Pad) | FCS | | 7 B | 1 B | 6 B | 6 B | 2 B | 4 B | 46–1500 B | 4 B |
Note:
- VLAN Tag is optional and present only in VLAN-enabled networks.
- The minimum Ethernet frame size is 64 bytes (including all fields).
- The maximum payload is 1500 bytes, and the maximum frame size (without VLAN) is 1518 bytes.