You don't have to be trained to participate. If you can only say zero foundation, it is more recommended to apply for classes. First, it is easier to learn and get a certificate faster. Second, you can also acquire more knowledge and learn more useful technologies.

Self-study like low-level HCIA may be more suitable, because the key point is to brush the questions, and more preparation can pass. Of course, you should also watch the teacher's video tutorial more, and I can share a set of basic versions for you:

Why are HCIE certificates so popular?

However, for example, taking the HCIP and HCIE exams, it is necessary to apply for classes systematically, because it involves many professional theories, which can not be solved by just brushing the questions and videos, especially HCIE, and it is still difficult to do experiments.

In addition, HCIE has the biggest change in difficulty and examination form. Judging from the difficulty, HCIE has greatly increased the depth of knowledge and inherited a large number of relatively new network technologies, so it is not easy to learn.