7.2 Link Aggregation Control Protocol (LACP) Explained
Key Concepts
Link Aggregation Control Protocol (LACP) is a protocol used to manage and control the aggregation of multiple physical links into a single logical link. Key concepts include:
- Link Aggregation: Combining multiple physical links into a single logical link to increase bandwidth and provide redundancy.
- LACPDU (LACP Data Unit): Messages exchanged between network devices to negotiate and manage link aggregation.
- Active and Passive Modes: Modes of operation for LACP, where Active initiates negotiations and Passive waits for negotiations.
- System Priority and System ID: Parameters used to determine the priority of devices in LACP negotiations.
- Link Aggregation Group (LAG): A group of physical links managed as a single logical link.
Link Aggregation
Link Aggregation combines multiple physical links into a single logical link, effectively increasing the bandwidth and providing redundancy. This is achieved by distributing traffic across the aggregated links, ensuring that if one link fails, the others can continue to carry the traffic.
Example: In a data center, two 1 Gbps links between a switch and a server can be aggregated into a single 2 Gbps link using LACP. This not only doubles the bandwidth but also ensures that if one link fails, the other can continue to operate.
LACPDU (LACP Data Unit)
LACPDU messages are exchanged between network devices to negotiate and manage link aggregation. These messages contain information about the device's capabilities, system priority, and other parameters necessary for establishing and maintaining link aggregation.
Example: When two switches are configured to use LACP, they exchange LACPDU messages to determine which links should be part of the aggregation group. These messages ensure that both switches agree on the configuration and can manage the aggregated links effectively.
Active and Passive Modes
LACP can operate in two modes: Active and Passive. In Active mode, a device actively initiates LACP negotiations. In Passive mode, a device waits for another device to initiate negotiations. Both modes are used to ensure that devices can establish link aggregation even if one device is configured to wait for the other.
Example: A switch configured in Active mode will send LACPDU messages to initiate link aggregation with another switch. If the other switch is in Passive mode, it will respond to the LACPDU messages and establish the aggregation group.
System Priority and System ID
System Priority and System ID are parameters used to determine the priority of devices in LACP negotiations. The device with the higher priority (lower numerical value) will have more influence in the negotiation process. System ID is typically the device's MAC address.
Example: If two switches have the same System Priority, the one with the lower System ID (MAC address) will have higher priority in LACP negotiations. This ensures that the negotiation process is deterministic and consistent.
Link Aggregation Group (LAG)
A Link Aggregation Group (LAG) is a group of physical links managed as a single logical link. LAGs are created and managed using LACP, and they provide increased bandwidth and redundancy. Each LAG has a unique identifier that is used to manage the aggregated links.
Example: In a network, a LAG with ID 1 might include two physical links between a switch and a server. Traffic is distributed across these links, and if one link fails, the other can continue to operate, ensuring continuous connectivity.