Collisions almost always happen when there is a speed or duplex configuration mismatch. For example, if a device is set to auto but it's connected to another device who is set manually to 100/Full, it is likely that collisions will happen.

Ideally both devices should be manually configured to their fastest common setting. Using 'Auto' on a congested link can cause an interface to revert down to 10/half and at the same time, cause some collisions.

When seeing collisions, always start by looking for a speed/duplex mismatch on the network

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