Layer 2 Switching: A Technical Deep Dive
Layer 2 switching operates at the Data Link Layer (Layer 2) of the OSI model, focusing on MAC addresses to forward Ethernet frames within a local area network (LAN). Unlike routers, which use IP addresses (Layer 3), Layer 2 switches build and maintain a MAC address table (CAM table) to efficiently direct traffic between devices on the same network segment. Below is a detailed breakdown of its mechanisms, protocols, and use cases:
- Core Functions of Layer 2 Switching
- MAC Address Learning:
- Switches dynamically learn MAC addresses by inspecting the source MAC of incoming frames and mapping them to the corresponding switch port.
- Example: When device A sends a frame to device B, the switch records A's MAC address and its ingress port in the CAM table.
- Frame Forwarding:
- Unicast: If the destination MAC is in the CAM table, the frame is forwarded only to the relevant port.
- Broadcast/Multicast: Frames are flooded to all ports except the ingress port (unless IGMP snooping is enabled).
- Unknown Unicast: Frames with unlearned MAC addresses are flooded to all ports.
- Loop Prevention:
- Uses protocols like Spanning Tree Protocol (STP) to block redundant paths and prevent broadcast storms.
- Key Components & Technologies
- MAC Address Table (CAM Table)
- Stores mappings of MAC addresses ↔ Switch Ports.
- Entries age out after a configurable time (default: 300 seconds).
- Example Table:
|
MAC Address
|
Port
|
VLAN
|
|
00:1A:2B:3C:4D:5E
|
Gi0/1
|
10
|
|
AA:BB:CC:DD:EE:FF
|
Gi0/2
|
20
|
- VLANs (Virtual LANs)
- Logically partitions a physical switch into multiple isolated broadcast domains.
- Traffic between VLANs requires Layer 3 routing.
- Link Aggregation (EtherChannel/LACP)
- Combines multiple physical links into a single logical link for redundancy and increased bandwidth.
- Uses protocols like LACP (802.3ad) to negotiate member ports.
- Security Features
- Port Security: Restricts MAC addresses allowed on a port.
- DHCP Snooping: Blocks rogue DHCP servers.
- Dynamic ARP Inspection (DAI): Prevents ARP spoofing.
- Layer 2 vs. Layer 3 Switching
|
Feature
|
Layer 2 Switching
|
Layer 3 Switching
|
|
Forwarding Basis
|
MAC addresses
|
IP addresses
|
|
Broadcast Domains
|
Single domain (unless using VLANs)
|
Multiple domains (routed networks)
|
|
Protocols
|
STP, VLANs, LACP
|
IP, OSPF, ACLs
|
|
Use Case
|
LAN segmentation, intra-VLAN traffic
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Inter-VLAN routing, WAN connectivity
|
- Key Protocols in Layer 2 Switching
- Spanning Tree Protocol (STP):
- Prevents loops by blocking redundant paths.
- Variants: RSTP (802.1w), MSTP (802.1s).
- LLDP (802.1AB):
- Discovers neighboring devices (e.g., switches, routers).
- VTP (VLAN Trunking Protocol):
- Synchronizes VLAN configurations across switches (largely deprecated in favor of manual configs).
- Benefits of Layer 2 Switching
- Low Latency: Hardware-based forwarding (ASICs) ensures near-wire-speed performance.
- Scalability: Supports large networks via VLANs and link aggregation.
- Cost-Effective: Simpler and cheaper than Layer 3 switches for intra-LAN traffic.
- Limitations
- Broadcast Storms: Requires STP to mitigate risks in redundant topologies.
- Limited Segmentation: VLANs provide isolation but require Layer 3 for inter-VLAN routing.
- No Routing: Cannot forward traffic between IP subnets without a Layer 3 device.
- Use Cases
- Enterprise LANs: Connect end-user devices (PCs, printers) within the same subnet.
- Data Centers: Build high-speed, low-latency server-to-server networks.
- VoIP/Video Conferencing: Prioritize real-time traffic using QoS on VLANs.
- Industrial Networks: Deploy deterministic, loop-free topologies with RSTP.
- Workflow Example: Frame Forwarding
- Frame Arrival: A frame from MAC A enters port Gi0/1.
- MAC Learning: Switch adds A → Gi0/1 to the CAM table.
- Destination Lookup:
- If MAC B is in the CAM table, forward to the mapped port.
- If MAC B is unknown, flood to all ports in the same VLAN.
- Loop Prevention: STP blocks alternate paths to avoid loops.
