LACP (Link Aggregation Control Protocol)

Within the IEEE specification, the Link Aggregation Control Protocol (LACP) provides a method to control the bundling of several physical ports together to form a single logical channel. LACP allows a network device to negotiate an automatic bundling of links by sending LACP packets to the peer (directly connected device that also implements LACP).

LACP Features and practical examples

  • Maximum number of bundled ports allowed in the port channel: Valid values are usually from 1 to 8.
  • LACP packets are sent with multicast group MAC address 01:80:c2:00:00:02 (01-80-c2-00-00-02)
  • During LACP detection period:
    • LACP packets are transmitted every second
    • Keep alive mechanism for link member: (default: slow = 30s, fast=1s)
  • LACP can have the port-channel load-balance mode:
    • link (link-id) Integer that identifies the member link for load balancing. The range is from 1 to 8.
  • LACP mode:
    • active: Enables LACP unconditionally.
    • passive: Enables LACP only when an LACP device is detected. (This is the default state)

Advantages Over Static Configuration

  • Failover occurs automatically: When a link fails and there is (for example) a media converter between the devices, a peer system will not perceive any connectivity problems. With static link aggregation, the peer would continue sending traffic down the link causing the connection to fail.
  • Dynamic configuration: The device can confirm that the configuration at the other end can handle link aggregation. With Static link aggregation, a cabling or configuration mistake could go undetected and cause undesirable network behavior.

Practical Notes

LACP works by sending frames (LACPDUs) down all links that have the protocol enabled. If it finds a device on the other end of the link that also has LACP enabled, it will also independently send frames along the same links enabling the two units to detect multiple links between themselves and then combine them into a single logical link. LACP can be configured in one of two modes: active or passive. In active mode it will always send frames along the configured links. In passive mode, however, it acts as “speak when spoken to”, and therefore can be used as a way of controlling accidental loops (as long as the other device is in active mode).

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