Xrhrft

I would like to set up a redundant link between two switches. I have four ports available to accomplish this.

Scenario A: Switch 1 has ports 1+2 trunked together via LACP. They are plugged into Switch 2 ports 1+2.

Scenario B: Switch 1 has ports 1+2 trunked together without LACP. They are plugged into Switch 2 Ports 1+2.

In Scenario B I can unplug one of the patch cables and the switches will still be linked, establishing redundancy. In Scenario A, what will happen when I unplug one of the patch cables? Will LACP allow the link over just one interface? Or will the link stop because an interface is missing.

Scenario A: Switch 1 has ports 1+2 trunked together via LACP. They are
plugged into Switch 2 ports 1+2.

Both links will be used, but a single flow will only use one link. There is a hashing algorithm that determines which flow uses which link. If one of the links goes down, then all the traffic will be switched to the other link. This happens very rapidly.

Scenario B: Switch 1 has ports 1+2 trunked together without LACP. They
are plugged into Switch 2 Ports 1+2.

They will not actually be trunked together. STP will block one link because it creates a single, loop-free path to the root bridge. When the active link goes down, STP will switch over to the redundant link, but this happens fairly slowly; a few seconds for RSTP, and up to 50 seconds for standard STP.

From a functional point of view, there's no difference between LACP trunks and static trunks. All links are aggregated (with the limitations Ron has already pointed out) and the aggregation group is redundant. An LACP trunk set up with eight ports works with anything between one to eight physical links - so does a static trunk.

The difference is that an LACP trunk only works when both sides negotiate the aggregation. Without successful negotiation the physical links fall apart into separate logical links. Usually, it's combined with a spanning tree protocol to avoid bridge loops - without STP the bridge loop would bring down the network.

In contrast, in a static trunk the links are aggregated when they're up. The switch doesn't check whether the trunk makes sense. You could use links terminated differently and you'd get weird and possibly unexpected effects.

Generally, LACP trunks are safer to use. You should only use static trunks when LACP isn't available.

Of course, the combination of LACP and STP calls for a better integrated solution that even works with multiple switches and meshed setups. This is what Shortest Path Bridging aka IEEE 802.1aq is for. Sadly it hasn't caught on in the mid-range class yet.