Etherchannel: Port Channel Discovery

When ports are added to a bundle the following configuration items must be identical:

  • Same speed and duplex
  • Same operating mode
  • If not trunking, same access VLAN
  • If trunking same trunk type, allowed VLANs and native VLAN
  • Each port in a port channel must have the same STP cost per VLAN on all links in the port channel
  • No ports can have SPAN configured

When a port channel is made an interface is also automatically added to the configuration, this interface inherits the configuration of the port it was added to.  If member port configurations differ the port channel will be in a suspended state and will not become working until the member whose configuration is different is corrected and identical to the port channel interface.

Any interface commands configured on the port channel interface are pushed down to the member links for conformity.  Here are the recommended guidelines for configuring port channels

  • do not create the interface port channel manually before bundling the ports, let the switch create it and populate its config automatically
  • When removing a port channel make sure to manually remote the interface port channel from the running config so that it’s config does not cause issues when a port channel with the same number may be created later
  • be sure to make the config of the physical ports identical before adding them to the port channel
  • if the physical ports config differs from the port channel interface config correct the port config first, only then proceed to perform changes to the port channel interface config.
  • port channel interface can either be a layer 2 or layer 3 depending on the physical bundled ports are configured for layer 2 or layer 3.  If needed you can change the type of interface it is after you’ve already created it.
  • Whenever resolving issues with err-disabled ports under a port channel be sure to shut down both the physical and port channel interfaces, then try to reactivate them, unbundling them and then recreating the port channel.

portchannel

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