I do not understand when will the core layer is needed. All the materials that I have read in general say that core layer should switch traffic quickly and no filtering. None say when or in what situations the core layer is implemented.
Network hierarchy model – Situation to implement core layer
design
Best Answer
For smaller deployments a Collapsed Core is usually fine. Collapsed Core means that some devices perform both Distribution and Core layer duties.
With just a few Access switches there's no need to build a large Core.
If you have a large network then you will have many Access switches. Let's say that you have 20 Distribution switches. Connecting all the Distribution switches would be a huge waste of ports and fiber and you would also need a lot of routing peerings. For everytime you add a new Distribution switch there would be a lot of work connecting it to the others and putting in new routing statements.
For those situations it makes sense to use a Core, you would then only add maybe two connections from each Distribution switch to the Core. The number of peerings on the Distribution layer switches would be a lot less as well.
The main factor is scalability but other things could affect as well like speeds and feeds, the distance your network is covering, external connections and factors like that.