What is Link Aggregation (LACP) and when should I use it?

Link Aggregation (IEEE 802.3ad, later 802.1AX) combines multiple physical links into a single logical link, providing both increased bandwidth and redundancy.

Key Concepts

• LACP (Link Aggregation Control Protocol): Dynamic negotiation
• Static aggregation: Manual configuration, no LACPDU exchange
• LAG (Link Aggregation Group): The bundled logical link
• Load balancing: By src/dst MAC, IP, or L4 port

Benefits

• Increased bandwidth (2-8x single link)
• Automatic failover on link failure
• Load distribution across links
• No spanning-tree recalculation needed

Use Cases

• Switch-to-switch trunks: Aggregation layer connections
• Server uplinks: High-throughput servers, NAS
• Core network bandwidth expansion

Limitations

• Individual flows do not exceed single link speed
• All member links must have same speed and duplex
• Cross-switch LAG requires MLAG/VPC (vendor-specific)

Configuration Tips

1. Use LACP over static for auto-recovery
2. Match load-balance algorithm to traffic pattern
3. Enable on both ends before connecting cables
4. Monitor with LACP status commands