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I recently had two switches from different vendors, one was an HP 5412 Chassis running ProCurve and the other is an Alcatel 4-Switch stack, have a LACP misconfiguration which brought down a portion of the network via a switching loop.

The LACP configurations were correct at first, but one of the switches (Alcatel stack) was restarted for maintenance, and the startup configuration was not saved correctly. This meant that the traffic was not handling 802.1q tagged packets on the correct VLANs and were all on the native VLAN (in this case, VLAN 2). The other side, however, attempted to negotiate LACP and, since there is no spannign tree within the network (not my design, I assure you), it created a switching loop.

My question is why LACP does not detect this mis-match and automatically disable the ports? Or rather why is it allowed to forward traffic if it has not negotiated a LACP LAG?

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    Aug 10, 2017 at 2:58

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[W]hy LACP does not detect this mis-match and automatically disable the ports?

Because LACP doesn't do that. If a port fails to setup a LAG, the ports function as normal ports. I am unaware of any switches that can be configured to "require" LACP.

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