Layer 2 Protocols and Technologies | Generated by AI
Question: What things/protocols typically run on Layer 2 (Data Link Layer) of the OSI model?
Answer:
The Data Link Layer (Layer 2) is responsible for node-to-node delivery of frames over a physical link, error detection/correction, and medium access control. The following protocols, technologies, and functions typically operate at Layer 2:
Common Layer 2 Protocols
- Ethernet (IEEE 802.3) – the most widely used LAN technology
- Wi-Fi (IEEE 802.11) – wireless LAN framing and medium access
- PPP (Point-to-Point Protocol) – used on serial links (e.g., WANs, dial-up)
- HDLC (High-Level Data Link Control) – serial link protocol
- Frame Relay – legacy WAN technology
- ATM (Asynchronous Transfer Mode) – cell-based Layer 2 protocol
- MPLS (Multiprotocol Label Switching) – often considered Layer 2.5, but labels are inserted at Layer 2
- LLDP (Link Layer Discovery Protocol) – neighbor discovery (Cisco uses CDP instead)
- STP (Spanning Tree Protocol) and its variants (RSTP, MSTP) – prevents Layer 2 loops
- LACP (Link Aggregation Control Protocol, part of 802.1AX/802.3ad) – EtherChannel/port-channel negotiation
- ARP (Address Resolution Protocol) – technically operates between Layer 2 and Layer 3 (resolves IP to MAC), but uses Layer 2 frames
Layer 2 Technologies & Concepts
- MAC addresses (48-bit hardware addresses)
- Switches and bridges – primary Layer 2 devices
- VLANs (802.1Q tagging)
- MAC address table (CAM table) learning and forwarding
- Frame encapsulation (Ethernet frames, Wi-Fi frames, etc.)
- Error detection (via FCS – Frame Check Sequence using CRC)
- Flow control (e.g., Ethernet pause frames)
Summary Table
| Category | Examples |
|---|---|
| Wired LAN | Ethernet (802.3) |
| Wireless LAN | Wi-Fi (802.11) |
| WAN/Serial | PPP, HDLC, Frame Relay |
| Switching protocols | STP, RSTP, LACP, LLDP/CDP |
| VLAN tagging | 802.1Q |
| Addressing | MAC addresses, ARP |
In short: Anything that deals with MAC addresses, frames (not packets), switches, and physical-link-specific framing is Layer 2.
Layer 3 (e.g., IP, ICMP, routing protocols) runs on top of these Layer 2 protocols inside the frame payload.