EtherChannel – Add Links to a Port Channel Group Without Downtime

etherchannelport-channelspanning treetrunk

There are two trunk links with the same configuration connecting two switches. Currently they are independent links and one is blocked with spanning-tree. I would like to convert these to a single port channel without any down time. Is this possible? What is the best procedure for doing so?

These are Cisco, but I think the concept would apply to any switches that support the same function.

What I've come up with so far:

  1. Shutdown the STP blocked interface, on the core side
  2. Add both interfaces on the down connection to the channel group
  3. No shutdown the interface that was shut down in step 1 (STP will block again)
  4. Shutdown the interface with STP forwarding active (STP will start forwarding on the port channel)
  5. Add the interfaces on the down connection to the channel group
  6. No shutdown the interface that was shutdown in step 4

The problem is that step 4 here creates about 1 second of outage. Is there a way to do this without any outage?

Best Answer

Short answer: no.

While the STP handover in step 4 or step 3 is usually faster than 1 second there's always a gap. Make sure you're using RSTP, MSTP or RPVST though. The original and largely obsolete STP is slow on convergence.

One tiny thing might reduce the gap a little further: before step 3, decrease the trunk's STP priority value below that of the yet active port. This will already initiate the handover in step 3, so it doesn't need to be a shutdown reaction but a more 'flowing' process.

As this seems to be very delicate connection I'd stage the exact process in a lab before doing it in production.

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