Networking - Link Aggregation (LACP)

Link Aggregation, also known as EtherChannel or Port Channel, is a technique used to combine multiple physical network links into a single logical link to increase bandwidth, provide redundancy, and improve network resiliency. The Link Aggregation Control Protocol (LACP), defined by IEEE 802.3ad (now 802.1AX), is the standard protocol used to automate the negotiation and management of these aggregated links.

1. Definition

Link Aggregation is the process of grouping two or more physical Ethernet links between switches, routers, or servers so that they act as one logical link for data transmission.

LACP is a dynamic link aggregation protocol that automatically detects, configures, and maintains these aggregated links between compatible devices.

In simpler terms:

Link aggregation increases the available bandwidth and provides redundancy by combining multiple cables into one logical connection.

2. Purpose of Link Aggregation

1. Increase Bandwidth:
   Multiple links are combined, providing higher throughput than a single link.

2. Redundancy and Fault Tolerance:
   If one physical link fails, traffic continues over the remaining links.

3. Load Balancing:
   Distributes traffic across all links in the aggregation based on MAC addresses, IP addresses, or port numbers.

4. Simplified Management:
   Aggregated links appear as a single logical interface for configuration.

3. LACP Overview

Link Aggregation Control Protocol (LACP) is part of the IEEE 802.3ad / 802.1AX standard.

• Purpose: Automatically detect, configure, and maintain link aggregation groups.

• Modes:

  • Active: Actively tries to form a link aggregation by sending LACP packets.

  • Passive: Responds to LACP packets but does not initiate aggregation.

• Default behavior: LACP exchanges LACP Data Units (LACPDUs) to negotiate link parameters.

4. Components of LACP

5. LACP Operation

1. Link Detection:
   Devices connected via multiple physical links exchange LACP Data Units (LACPDUs) to detect compatible links.

2. Negotiation:
   LACP determines which links can be aggregated based on speed, duplex, and configuration parameters.

3. Aggregation Formation:
   Selected links are grouped into a single logical interface called a LAG (Link Aggregation Group).

4. Traffic Distribution:
   Packets are distributed across links based on load-balancing algorithms (e.g., source/destination MAC or IP hashing).

5. Monitoring:
   LACP continuously monitors links. If a link fails, it is removed from the LAG, and traffic is redistributed across the remaining links.

6. LACP Frame Format

LACP frames (LACPDUs) are sent as Ethernet control frames with the following key information:

• Actor Information: System ID, Port ID, Port Priority

• Partner Information: System ID, Port ID, Port Priority

• Operational Key: Identifier for the LAG group

• LACP State: Flags indicating link status (active, passive, aggregation)

7. LACP vs Static Link Aggregation

8. Load Balancing in LACP

Traffic distribution depends on hashing algorithms, which vary by vendor:

• Based on MAC addresses: Source MAC, Destination MAC, or both.

• Based on IP addresses: Source IP, Destination IP, or both.

• Based on TCP/UDP ports: For more granular balancing.

Important: LACP does not split a single flow across multiple links; it balances multiple flows across links.

9. Maximum Number of Links

• IEEE 802.3ad / 802.1AX supports up to 16 physical links per LAG.

• Cisco typically supports up to 8 active links per EtherChannel, with additional standby links if configured.

10. Example Scenario

Imagine two switches connected with four 1 Gbps links:

• Without aggregation: Each link is independent (1 Gbps per link).

• With LACP: All four links combine into one 4 Gbps logical link, with redundancy.

If one link fails:

• Traffic is redistributed across the remaining three links.

• No downtime occurs for ongoing sessions.

11. Configuration Example (Cisco IOS)

On Switch A

SwitchA(config)# interface range gi0/1 - 4
SwitchA(config-if-range)# channel-group 1 mode active
SwitchA(config-if-range)# exit
SwitchA(config)# interface port-channel 1
SwitchA(config-if)# switchport mode trunk

On Switch B

SwitchB(config)# interface range gi0/1 - 4
SwitchB(config-if-range)# channel-group 1 mode passive
SwitchB(config-if-range)# exit
SwitchB(config)# interface port-channel 1
SwitchB(config-if)# switchport mode trunk

Explanation:

• channel-group 1 mode active/passive — Configures LACP negotiation.

• interface port-channel 1 — Configures the logical LAG interface.

• switchport mode trunk — Allows VLAN traffic to pass over the aggregated link.

12. Advantages of LACP

13. Disadvantages of LACP

14. LACP vs 802.1Q Trunking

• 802.1Q handles VLAN tagging to carry multiple VLANs over a single link.

• LACP handles link aggregation to increase bandwidth and provide redundancy.

• Often used together: An EtherChannel (LACP) can carry multiple VLANs using 802.1Q trunking.

15. Real-World Applications

• Data Centers: Aggregating multiple uplinks between top-of-rack and core switches.

• Enterprise Networks: Connecting access switches to distribution switches for higher bandwidth.

• Servers: NIC teaming with LACP for higher throughput and redundancy.

• Service Providers: Aggregating links for backbone network resilience.

16. Summary

17. Conclusion

Link Aggregation (LACP) is a critical networking technique to maximize bandwidth, ensure redundancy, and simplify management of multiple physical links. By dynamically negotiating link groups, LACP provides both high availability and scalable performance, making it a core feature in enterprise networks, data centers, and high-performance computing environments.