Recipe: Etherchannel

I have recently started writing recipes for things that are on the CCIE blueprint. For more details, see the reference series page. This is a short recipe to configure etherchannels.

Prerequisites

First, we need to gather the following information:

  • Which interfaces are involved in the Etherchannel
  • PAGP, LACP or ON?
    –> Does one side need to start the negotiation?
  • What is the final configuration of the port?
    –> L2 or L3
    –> 802.1q settings
    –> Spanning tree settings

Steps L2

The following steps should be done on both sides:

  • Foreach interface: Default [interface]
  • interface range [interfaces]
  • shut
  • switchport
  • 802.1q settings
  • spanning-tree settings
  • channel-group [NUMBER] mode [MODE]

After finishing the previous steps on both devices, we unshut the interfaces on both sides:
* no shut

Now we should see the etherchannel come up and we can go to verification.

Steps L3

The following steps should be done on both sides:

  • Foreach interface: Default [interface]
  • interface range [interfaces]
  • shut
  • no switchport
  • channel-group [NUMBER] mode [MODE]

After finishing the previous steps on both devices, we unshut the interfaces on both sides and configure L3:

  • no shut
  • int po
  • ip address address
  • no shut

We should now see the etherchannel come up and we can go to verification.

Verification

  • Shows the portchannel as RU or SU (L3 or L2 respectively)
  • Shows all member interfaces as P

For L2:
Show spanning-tree shows only the etherchannel, and none of its member ports.
For L3:
ping [NEIGHBOR] gives replies

Troubleshooting

If things don’t come up, there could be a few problems:

  • Interfaces are shutdown on either side. show etherchannel summary shows D on shutdown interface and w on the non-shutdown side.
  • Non-compatible channel-group modes. Both sides are passive, or one side is PAgP and the other is LACP.
  • Interfaces don’t have the same L2 config as each other or the portchannel interface.
  • Wrong interface configured: If the etherchannel is configured facing the wrong neighbor, it will not negotiate. Check this by running show cdp neighbor and show lacp/pagp neighbor.

3 comments

  1. This protects your environment from potential topology loops, as it s safe to assume that if you see a BPDU flow into a port that should be facing a server, something has gone badly wrong.

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