Cisco LACP

LACP (Link Aggregation Control Protocol) is a standard protocol to bundle multiple physical links between two devices into a single logical link, increasing the available bandwidth and providing link redundancy. When configuring LACP, you can choose between Active and Passive modes. Additionally, LACP supports two transmission rates: Fast and Slow. Let’s explore the differences between these options:



  1. Active Mode:
  • In Active mode, the device actively initiates the LACP negotiation process by sending LACP packets to the connected device.
  • The device in Active mode will continue sending LACP packets even if it doesn’t receive any response from the other device.
  • Active mode is typically used on devices capable of aggregating multiple links and wants to form a link bundle actively.
  1. Passive Mode:
  • In Passive mode, the device waits for the other device to initiate the LACP negotiation process.
  • The device in Passive mode does not send LACP packets unless received first from the other device.
  • If the Passive mode device does not receive any LACP packets, it assumes that the other device does not support LACP.
  • Passive mode is typically used on devices that cannot initiate LACP negotiation or devices that want to be more responsive to the negotiation requests from the other end.


Transmission Rates

LACP supports two transmission rates: Fast and Slow.

  1. Fast transmission rate means that LACP packets are sent more frequently, resulting in quicker detection and recovery from link failures or changes.
  2. Slow transmission rate means that LACP packets are sent at a lower frequency, which may result in slightly slower detection and recovery times.

The choice of transmission rate depends on the specific network environment and requirements. Fast transmission rate is typically preferred for most scenarios.

It’s important to note that both devices involved in the LACP link aggregation must be configured correctly for both the modes and transmission rate (Fast or Slow) for successful negotiation and link aggregation to occur.


Mode Compatibilities

When configuring Link Aggregation Control Protocol (LACP), you have three options:

  1. Active: In this mode, the port actively communicates its ability to form an aggregated link using LACP, sending LACP packets.
  2. Passive: In this mode, the port can form an aggregated link using LACP if requested, but it does not actively communicate this.
  3. On: This mode is also known as static Link Aggregation or manual mode. It forces the device to form an aggregated link without any LACP negotiation. Both ends must be manually configured to agree on the formation of the link aggregation group.

Here are the combinations of these modes and whether they can form a channel:

  • Active/Active: Works. Both ends will actively negotiate the formation of a link aggregation group using LACP.
  • Active/Passive: Works. One end actively negotiates the LACP link and the other end passively accepts the negotiation.
  • Passive/Passive: Does not work. Neither end will start LACP negotiation, so no aggregated link is formed.
  • Active/On: Does not work. The ‘On’ end does not use LACP and therefore will not respond to LACP packets from the ‘Active’ end.
  • Passive/On: Does not work. The ‘Passive’ end waits for LACP packets which the ‘On’ end will never send.
  • On/On: Works, but without LACP negotiation. Both ends must be manually configured to match each other. This method does not provide the same level of fault detection as Active/Active or Active/Passive.

Keep in mind that when you’re configuring LACP, it’s generally considered best practice to use Active/Active or Active/Passive combinations to allow for the highest level of fault detection and dynamic configuration.


Importance of Using ON

The “On/On” configuration in link aggregation essentially involves statically or manually configuring both ends of the link to form an aggregated link, bypassing the Link Aggregation Control Protocol (LACP) negotiation process.

This mode is also known as static Link Aggregation or manual mode. In this mode, the two devices involved in the link aggregation are configured to treat certain physical ports as a single logical interface. This is done by manually specifying the same group of ports on both devices that are to be used for the aggregated link.

One thing to note here is that, while this configuration can work if set up correctly, it doesn’t provide any of the benefits of dynamic configuration or fault detection that you get with LACP. In an “On/On” configuration, if a physical link within the aggregation goes down, the devices won’t be able to dynamically adjust to the change. Similarly, if there is a misconfiguration on one end of the link, the devices won’t be able to detect and correct it automatically.

Furthermore, because “On/On” mode bypasses LACP, there’s no protocol-based error checking to ensure that both sides of the connection are correctly set up for aggregation. If one side is set for aggregation and the other is not, network loops can occur.

Therefore, while the “On/On” configuration can technically work, it’s generally recommended to use LACP (“Active/Active” or “Active/Passive“) for link aggregation whenever possible. LACP provides additional protection against misconfigurations and faults, and it allows the link to dynamically adjust to changes in the physical links.