EtherChannels

PAgP Configuration

Sections:

Overview:

  • PAgP is a Cisco proprietary EtherChannel standard that is used across Cisco environments and operates the very same way as the open standard LACP standard
  • PAgP uses two negotiation modes to form an EtherChannel and consist of the following:

PAgP Modes of Operation

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

Cisco switch example of the supported available EtherChannel modes 

Lab Topology

Scenario: In this lab exercise, we will configure interfaces Gi1/0/25 and Gi1/0/27 from the HQ-CoreSW1 switch to be defined in a PAgP EtherChannel bundle towards the HQ-DistroSW2 switch on ports F0/2 and F0/4.

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

After defining the EtherChannel on the neighbor switch (HQ-DistroSW2) now it is time to bring up the EtherChannel port (po2) of both switches.

Note: 

  • Configuration changes done on the EtherChannel interface will apply to all the physical ports in the bundle
  • In this example, bringing up the EtherChannel interface will also bring up the physical ports Gi1/0/25 and Gi1/0/27 

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 bundled 
  • Other key statistics is the protocol the channel is operating on including the port channel number 

Show etherchannel 2 port-channel

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

Show etherchannel 2 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
  • Another great feature of this command with PAgP configurations is the ability to analyze the partner port information of the remote side of the EtherChannel link

Scenario2: When EtherChannel is not Working

Notes:

  • When a port(s) are having issues bundling into an EtherChannel, it is essential to refer back to the matching parameter pre-requisites list to ensure the physical ports are sharing the same settings to successfully bundle in the EtherChannel
  • In this lab exercise, I've purposely changed the interface speed of the Gi1/0/25 interface to 10Mbps causing the port to go into a suspended state in the PAgP EtherChannel

Notes:

  • Issuing a more detailed command like 'show etherchannel 2 detail' can help spot any error messages associated with an unbundled physical port 
  • In this example, this command gives insights into the probable cause of a speed mismatch on port Gi1/0/25 causing it to go into a suspended state