I think learning the basics of subnetting, vlaning, switching, and routing and IOS were the most important parts of it. I don’t think I ever needed to really know how to configure a T1 or various other dead technology. But pretty much every vendor uses a clone of IOS and knowing the proper way to logically configure a network is important. I’ve seen so many abhorrent hacked together networks in my day.

I worked for an ISP who paid for mine back in the day. Took like a 3 day e-learning course and then examine. The real world experience was probably the best way to learn