Link Aggregation Control Protocol (LACP) Explained

An EtherChannel is a group of links that work together as a single link. Link Aggregation Control Protocol (LACP) is an open-standard protocol for EtherChannel Configurations. It dynamically adds and manages ports in the EtherChannel. This tutorial explains how to configure, verify, and manage LACP on Cisco switches.

LACP EtherChannel Configuration

The following interface configuration mode command creates a new EtherChannel.

Switch(config-if)#channel-group [number] mode [mode type]

A switch supports up to six EtherChannels. Hence, you can use any number from 1 to 6 here. If an EtherChannel with the specified number exists, it adds the interface to that EtherChannel. If not, it creates a new EtherChannel and adds.

There are five modes you can use with this command. The following table lists them.

For LACP EtherChannel configuration, you have two modes: active and passive. If you configure the active mode, the switch enables LACP EtherChannel and adds the interface. If you configure the passive mode, it listens for LACP messages from the other side. If it receives LACP messages, it enables the LACP EtherChannel and adds the interface to it. You must run the above command on all interfaces you want to add to the EtherChannel.

Packet Tracer Example

Either create a Packet Tracer lab as shown in the following image, or download the following pre-created LAB and open it in Packet Tracer.

Download the Packet Tracer LAB

This network has two switches connected with four links. If two switches have more than one connection, they automatically enable only one connection while keeping the remaining ones disabled.

You can verify it by viewing the STP port states. The show spanning-tree command displays the STP port states.

STP blocked F0/2, F0/3, and F0/4 ports to remove loops. The switch can not use these ports to forward frames. Access the interface configuration mode of F0/1, F0/2, F0/3, and F0/4 interfaces and run the following command.

Switch(config-if)#channel-group 1 mode active

You can use any channel number between 1 and 6. The channel number must be the same across all interfaces you want to add to a channel group. You can add up to eight ports to a single EtherChannel.

As mentioned earlier, you must configure one side of a link to be in active mode. You can configure the other side in active or passive mode.

Access the interface configuration mode of F0/1, F0/2, F0/3, and F0/4 interfaces on the second switch and run the following command.

Switch(config-if)#channel-group 1 mode active

Packet Tracer lab with LACP Configuration

Verifying LACP

There are many ways to verify the LACP configuration. After the LACP configuration, STP treats all links in the pool as a single link. You can use the show spanning-tree command to verify this.

As the above output shows, there is only one interface instead of four. It is a pool of all four interfaces. STP treats this pool as a single interface. If any interface from this pool fails, it does not trigger STP. STP reruns its algorithm only when all interfaces inside the pool fail.

Use the show etherchannel port-channel command to view detailed information about the EtherChannel.

Use the show etherchannel summary command to view the ports and their states in each etherchannel.

This pool has four ports. The output also lists each port's state in the pool. The P indicates that the port has passed all necessary configuration checks.

Conclusion

Link Aggregation Control Protocol (LACP) is an EtherChannel protocol. It is an open-standard protocol. It works only on all vendor switches. It allows us to add up to sixteen ports in an EtherChannel. However, it activates only eight ports at a time, leaving the remaining ports in the waiting state. If an active port fails, the first waiting port is activated. It supports two modes: active and passive. In active mode, it unconditionally enables the EtherChannel on the port and sends LACP messages from the port. In passive mode, it listens for LACP messages. If it receives LACP messages from the other side, it compares the message's parameters with the port's parameters. If both match, it enables LACP on the port. A switch can have six EtherChannels. An EtherChannel can have up to eight ports.