LACP Configuration

Sections:

• Lab Topology
• LACP Configuration
• Troubleshooting 

Overview:

• LACP is an open standard protocol and is the most common deployment method due to its multivendor support
• LACP uses two negotiation modes to form an EtherChannel and consist of the following:

LACP Modes of Operation

• Active
  • Setting Active mode on one side of the logical link will proactively attempt to form an EtherChannel with the other side of the link
  • Active mode will peer with neighbors configured as either Active or Passive
  • It is common to configure both sides of the logical link as active for simplicity 
• Passive
  • Setting Passive mode on one side of the logical link will passively wait to form an EtherChannel with the other side of the link
  • Passive mode will only peer with neighbors configured as Active 
  • Passive/Passive configurations will not form an EtherChannel

Cisco switch example of the supported available EtherChannel modes 

Lab Topology

Scenario: In this lab exercise, we will discuss the commands to establish an LACP EtherChannel. Interfaces Gi1/0/21 and Gi1/0/23 from the HQ-CoreSW1 switch will be configured with an LACP EtherChannel towards the HQ-DistroSW1 switch on interfaces F0/1 and F0/3.

LACP Configuration

Note: Configuring ports in an EtherChannel can be accomplished when all ports are in an Up/Up or Down/Down state

• It is generally recommended to shutdown the physical ports before defining an EtherChannel for the following reasons:
  • Avoids interface/port flapping
  • Ensures a smooth negotiation process
  • Prevents real-time traffic loss if Up/Up ports are actively being defined in an EtherChannel
  • Prevents configuration mismatch or conflicts 

LACP/PAgP/Static bundles all use a channel group number to define an EtherChannel.

Note:

• On a switch or router, all local port members in an EtherChannel need to be configured with the same group number 
• However, channel group numbers can be different across switches
  • Example: Switch (A) using channel group 1 & Switch (B) using channel group 2 can form an EtherChannel relationship
• It is best practice to use the same channel group number between switches in an EtherChannel relationship for simplicity

After defining the EtherChannel group number to enable LACP on the neighbor switch (HQ-DistroSW1) now it is time to bring up the LACP EtherChannel port (po1) of both switches.

Note: 

• Configuration changes done on the EtherChannel interface will apply to all physical ports in the bundle
• In this example, bringing up the EtherChannel interface will also bring up the physical ports Gi1/0/21 and Gi1/0/23 

Troubleshooting Show Commands

Scenario1: When EtherChannel is Working

Show etherchannel summary

• Most common show command to verify if the configured EtherChannel is operating correctly 
• Referring to the Flag key, physical ports showing a key 'P' states that the ports are actively bundled 
• Other key statistics is the protocol the channel is operating on including the configured port channel number 

Show etherchannel 1 port-channel

• Specifying the port channel number in the command, this command displays more detailed statistics of the EtherChannel interface including the physical ports in the bundle and the overall EtherChannel bundle uptime  

Show etherchannel 1 detail

• This command shows a more detailed view of each physical port in the EtherChannel bundle including the port state if any issues are present with the bundling of the physical port

Scenario2: When EtherChannel is not Working

When a port(s) are having issues bundling into the EtherChannel, it is essential to refer back to the matching parameter pre-requisites list to ensure all ports are sharing the same settings to successfully join the active bundle

Issuing a more detailed command like 'show etherchannel 1 detail' can help spot any error messages associated with an unbundled port.