LACP EtherChannel

LACP EtherChannel is an open-standard protocol defined by IEEE 802.3ad. It allows switches from different vendors to automatically bundle multiple physical links into a single logical connection.

Unlike Static EtherChannel, which relies on manual configuration on both sides, LACP automatically negotiates which interfaces can form the bundle.

How LACP Works

Each interface participating in an LACP EtherChannel operates in one of two modes:

• Active mode → the switch initiates the negotiation process.

• Passive mode → the switch waits for an LACP request before joining the bundle.

Figure 1 – LACP EtherChannel Using Passive and Active Modes

When one switch runs in Active mode and the other in Passive mode, the EtherChannel forms successfully.
If both sides are Passive, no negotiation starts, and the aggregation does not form.

For example, if SW1 uses Passive mode and SW2 uses Active mode, they automatically create a Port-Channel, grouping several physical interfaces into one logical link.

Before You Begin

Before configuring LACP EtherChannel, make sure all participating interfaces meet the following conditions:

• Interface consistency: all interfaces must operate with the same speed, duplex, and type.

• Trunking or VLAN alignment: for Layer 2 operation, interfaces must share identical VLAN or trunk configurations.

• Operational status: each physical link must be active.

Figure 2 – LACP Link Requirements

If any of these conditions are not met, the EtherChannel will fail to form even with LACP enabled.
Once your interfaces are verified and consistent, you can move on to the configuration phase.

For this configuration, we will use GigabitEthernet interfaces running in full-duplex mode, and we will allow VLAN 50 on the trunk link.

In this section, we configure an LACP EtherChannel by enabling LACP negotiation, configuring the logical link as a trunk, and allowing VLAN 50 for demonstration purposes.

Figure 3 – LACP Configuration Between Passive and Active Modes

Configuration on SW1

Step 1: Enter Interface Configuration Mode

Select the physical interfaces that will form the EtherChannel.

SW1(config)# interface range g0/0-1

Step 2: Enable LACP (Passive Mode)

LACP is enabled using passive mode, meaning the switch waits for LACP negotiation packets from the remote switch.

SW1(config-if-range)# channel-group 1 mode passive
Creating a port-channel interface Port-channel 1
SW1(config-if-range)# exit

After this command, a logical interface called Port-Channel 1 (Po1) is automatically created.

Step 3: Configure the Port-Channel as a Trunk

Configure the logical interface as an 802.1Q trunk and allow VLAN 50.

SW1(config)# interface port-channel 1 
SW1(config-if)# switchport trunk encapsulation dot1q
SW1(config-if)# switchport mode trunk 
SW1(config-if)# switchport trunk allowed vlan 50

These commands configure the Port-Channel as a trunk link and allow VLAN 50 to pass through the aggregated connection between the switches.

Configuration on SW2

Now, we apply the same EtherChannel principle, but this time SW2 is configured to operate in LACP active mode.
In active mode, the switch actively sends LACP packets to initiate and negotiate the EtherChannel.

SW2(config)# interface range g0/0-1
SW2(config-if-range)# channel-group 1 mode active
Creating a port-channel interface Port-channel 1
%LINK-5-CHANGED: Interface Port-channel1, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface Port-channel1, changed state to up
SW2(config-if-range)# exit

Configure the logical Port-Channel interface as an 802.1Q trunk and allow VLAN 50.

SW2(config)# interface port-channel 1 
SW2(config-if)# switchport trunk encapsulation dot1q
SW2(config-if)# switchport mode trunk 
SW2(config-if)# switchport trunk allowed vlan 50

Once these steps are complete, both switches have successfully established an LACP EtherChannel, ready for verification.

Now that both switches are configured, let's verify that the LACP EtherChannel is active and correctly passing VLAN 50 traffic.

You'll first check that the EtherChannel has formed successfully, then confirm that trunking is enabled on the Port-Channel interface.

Figure 4 – Verifying LACP EtherChannel Configuration

Step 1 – Verify EtherChannel Formation

Use the following command on each switch to confirm that the EtherChannel is up and operational:

SW1

SW1# show etherchannel summary
Flags:  D - down        P - in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      f - failed to allocate aggregator
        u - unsuitable for bundling
        w - waiting to be aggregated
        d - default port

Group  Port-channel  Protocol    Ports
------ ------------- ----------- ----------------------------------------------
1      Po1(SU)       LACP        Gi0/0(P) Gi0/1(P)

Here's how to interpret the result:

• Po1(SU) shows that Port-Channel 1 is active in Layer 2 mode (S) and currently up (U).

• LACP confirms that the bundle is formed dynamically using the Link Aggregation Control Protocol.

• Gi0/0(P) Gi0/1(P) indicate that both interfaces are successfully participating in the EtherChannel.

Repeat the same command on SW2 to confirm that both switches show the same operational status.
If all flags match, the EtherChannel has been correctly established.

SW2

SW2# show etherchannel summary
Flags:  D - down        P - in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      f - failed to allocate aggregator
        u - unsuitable for bundling
        w - waiting to be aggregated
        d - default port

Group  Port-channel  Protocol    Ports
------ ------------- ----------- ----------------------------------------------
1      Po1(SU)       LACP        Gi0/0(P) Gi0/1(P)

This confirms that your LACP EtherChannel configuration is working as intended with both switches dynamically negotiating and bundling the link!

Step 2 – Verify Trunk Configuration

Next, check that the Port-Channel interface is configured as a trunk and that VLAN 50 is allowed.

SW1

SW1# show interfaces trunk
Port        Mode         Encapsulation  Status        Native vlan
Po1         on           802.1q         trunking      1

Port        Vlans allowed on trunk
Po1         50

This output confirms that:

• The 802.1Q encapsulation method is active.

• The Port-Channel interface operates in trunk mode.

• Only VLAN 50 is allowed to pass through the trunk.

Run the same command on SW2. You should see identical results.
If both sides display Po1 trunking with VLAN 50, the configuration is complete and fully functional.

Repeat the command on SW2:

SW2

SW2# show interfaces trunk
Port        Mode         Encapsulation  Status        Native vlan
Po1         on           802.1q         trunking      1

Port        Vlans allowed on trunk
Po1         50

The same output confirms that trunking is correctly configured on both switches and that VLAN 50 is allowed.

Step 3 – Validate Connectivity

To confirm the setup works, connect one device to each switch and assign them to VLAN 50.
Then, test communication between them using ping.
If the ping succeeds, the EtherChannel and trunk are operating correctly.
If it fails, review the interface consistency, VLAN assignment, and trunk configuration before troubleshooting further.

You have now completed the LACP EtherChannel configuration between the two switches.
The physical links are aggregated into a single logical connection, improving both stability and efficiency.
Trunking is correctly configured, allowing VLAN 50 traffic to pass through the Port-Channel interface.

Figure 5 – Final LACP EtherChannel Summary

With LACP, link aggregation is managed dynamically, reducing manual errors and providing automatic recovery if a physical link fails.
Because it follows an open standard, this method ensures interoperability between equipment from different vendors.

Key Takeaways

• LACP (IEEE 802.3ad) automatically forms EtherChannels and minimizes configuration errors.

• Verify interface consistency (speed, duplex, VLAN) before enabling LACP.

• Confirm the trunk configuration on the Port-Channel so that VLAN traffic can pass correctly.

• Always test connectivity between devices in the same VLAN to validate the setup.