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Does disabling LACP affect service?

Hi all, I tried to continue existing similar topic but perhaps it is better to start a new one.
I have 2x VSP7200 stacked switches connected with 4x10G ports in a LAG to a Cisco VHS cluster, at Cisco end it is a single PortChannel. Now we need to add new VLANs to this LAG and for that we have to disable and re-enable LACP on both VSP switches. Is this service-affecting? Obviously it will happen out of hours, but if it is affecting then the change window and amount of regression testing on the apps side, and overall perceived risk will be much larger.

Best answer by Paul A. Leroux 25 March 2019, 13:09


however I would still do this inside a maintenance window.
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Userlevel 1
For future reference, in the past on certain platforms disabling LACP on a link was the only way it was possible add or remove a VLAN from an active LACP LAG. However, this had been changed on recent VSP/VOSS code (sometime in the 5.x timeframe I believe).

You have probably already found that you cannot add a vlan to active ports on an active LACP link but you can add the VLAN to the MLT.

To add a VLAN to an active LACP link you can add it to the MLT ID that the LACP key is bound to. Example:To add VLAN 99 to MLT 1 you enter "vlan mlt 1". If you run the 'show running-config' command after this you will see the vlan was added to the individual ports. This can be done without having to disable the link or LACP on the link. Please understand that if there are any inactive links that are members of the LACP link at the time you run this you must also add the vlan directly to the inactive port (vlan member add 99 1/1) as described above.
Userlevel 4
@IgorN as mentioned, LACP failover is not sub-second.
Hi Paul, I planned this change and it was executed by other people, they told me that they followed the script, but I can 't assure you. In the aftermath, I saw TCP teardown and re-establishment in the application logs which coincided with the change window.
Userlevel 4
@IgorN are you saying that when you took down two links of the 4 link LACP/MLT you lost connectivity?
For those who will find this topic later, in real change we observed TCP links re-established between the two sides of the LAG, 4 links in the group were brought down two after two. Not sure though why this happened, TCP should be tolerant to the network connectivity gap whithin LACP convergence.time
Userlevel 4

If it was all SPBm or MLT/SMLT it would be a different story. I have customers that would do that change in the middle of the day. However, from my experience LACP failover is not sub-second.
Great, thanks! As I said, this will be out of hours, obviously during scheduled maintenance window.
Userlevel 4

however I would still do this inside a maintenance window.
Thanks Paul! Do you think this sequence is viable to avoid service interruption?
VSP-1: shutdown 2 interfaces > (LAG works on 2 links on VSP-2) > disable LACP > add VLANs > enable LACP > enable interfaces
VSP-2: shutdown 2 interfaces > (LAG works on 2 links on VSP-1) > disable LACP > add VLANs > enable LACP > enable interfaces
Userlevel 4
Yes, you will see a service impact because disabling LACP can create a loop.

I would disable the port on one side of the network and force a failover to the single link.
You can now disable LACP, make your vlan change, re-enable LACP and re-endable the port.

repeat for the other side of the network.