Configuring LACP EtherChannels

LACP: The Industry Standard for Link Aggregation

While Cisco has its own proprietary protocol (PAgP), the industry has largely adopted the open standard LACP (IEEE 802.3ad). This ensures interoperability between devices from different vendors. LACP dynamically negotiates the creation and maintenance of an EtherChannel by exchanging LACP packets between switches.

To get a clear overview of LACP and how it compares to other methods, let's start with a short video segment.

LACP Negotiation Modes

LACP uses two main modes to negotiate whether an EtherChannel should be formed:

• Active: In this mode, a port actively sends LACP packets to its neighbor, initiating negotiations to form an EtherChannel.
• Passive: In this mode, a port does not initiate LACP negotiation. It only responds to LACP packets it receives from a neighbor.

For an EtherChannel to be successfully formed, the modes on the connected ports must be compatible. A channel will form if the combination is:

• Active – Active
• Active – Passive

However, a channel will not form if both sides are set to Passive, as neither port will ever initiate the negotiation.

For simplicity and reliability, it's a common best practice to configure both sides as Active.

Configuration Prerequisites

Before you start bundling interfaces, it's critical to ensure they are configured identically. If the settings don't match, the channel will fail to form correctly. All physical interfaces within an EtherChannel bundle must have the same:

• Speed and Duplex settings (e.g., all 1 Gbps, full-duplex).
• Switchport Mode (either all access or all trunk).
• VLAN Configuration:
  • If they are trunk ports, they must have the same native VLAN and the same list of allowed VLANs.
  • If they are access ports, they must be assigned to the same access VLAN.

Configuring a Layer 2 LACP EtherChannel

Configuring an LACP EtherChannel is a two-step process: you first assign the physical interfaces to a channel group, which creates a logical port-channel interface. Then, you apply your Layer 2 configurations to that logical interface.

Let's walk through the commands.

Step 1: Assign Physical Interfaces to a Channel Group

First, you need to select the physical interfaces you want to bundle and assign them to a channel group using the channel-group command. We'll use the interface range command to configure multiple ports at once, which is an efficient practice.

Switch(config)# interface range GigabitEthernet0/1 - 2
Switch(config-if-range)# channel-group 1 mode active

Let's break down that command:

• channel-group 1: This command adds the interfaces to a channel group. The number (1 in this case) identifies the group. This number is locally significant, meaning it doesn't have to match on the neighboring switch, but all interfaces on this switch that are part of the same bundle must be in the same group. This command automatically creates the logical Port-channel1 interface.
• mode active: This sets the LACP mode to active, causing the ports to actively negotiate the EtherChannel.

Step 2: Configure the Logical Port-Channel Interface

After creating the channel group, all Layer 2 configurations (like setting up a trunk) should be applied to the logical port-channel interface, not the individual physical ports. This ensures consistency across all links in the bundle.

Switch(config)# interface port-channel 1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk native vlan 100
Switch(config-if)# switchport trunk allowed vlan 10,20,30

You would then apply a matching configuration on the neighboring switch.

The following resource provides a step-by-step walkthrough of this exact process.

Demonstration and Verification

Watching a live demonstration can help solidify these steps. The CertBros video below offers a concise and practical config example.

The show etherchannel summary command gives you a quick, high-level view of all port channels on the switch. A healthy Layer 2 LACP EtherChannel will look like this:

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)       LACP        Gi0/1(P)    Gi0/2(P)

Here's how to interpret the flags:

• Po1(SU):
  • S: Indicates this is a Layer 2 EtherChannel.
  • U: Means the port channel is in use (i.e., it's up and running).
• Gi0/1(P):
  • P: Means the port is bundled in the port-channel and is operating correctly.

If you see these flags, your EtherChannel is working! Any other flags (like D for down or s for suspended) indicate a problem, which we will cover in a future lesson on troubleshooting.

Conclusion

In this lesson, you have moved from theory to practice by learning how to configure a Layer 2 EtherChannel with LACP. This is a fundamental skill for building resilient and high-bandwidth links between core devices in your network designs.

Key Takeaways:

• LACP is the industry-standard protocol for negotiating EtherChannels and uses active and passive modes. To form a channel, at least one side must be active.
• Configuration is a two-part process:
  1. Bundle physical interfaces using the channel-group <number> mode active command.
  2. Apply Layer 2 settings (like trunking) to the newly created logical interface port-channel <number>.
• All physical interfaces in a bundle must have matching configurations (speed, duplex, VLANs).
• The primary command to verify the status of your EtherChannel is show etherchannel summary. Look for the (SU) and (P) flags to confirm it is operational.

In our next lesson, we will dive deeper into verification by exploring additional show commands that provide more detailed information. We'll also begin to look at how to interpret different status flags to troubleshoot common EtherChannel configuration issues.