I am new to networking and currently I have been learning about three layer model from Cisco. All sources say that Core layer is the one providing high speed switching and it is at the top of the hierarchy. But does it mean there is nothing about core layer?

I designed a network where there are two switches below the IPS and routing and I think core layer is just these two switches..or is it everything above that as well?!

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  • What do you mean by "there is nothing about core layer?" Apr 24, 2014 at 10:37
  • I mean that in all diagrams and examples I have seen, the core layer is always at the very top, having nothing above. In my case, I put firewalls and routers to ISP above. But have no idea if this is correct.
    – Pietros
    Apr 24, 2014 at 10:48
  • 1
    your firewalls, ISP uplinks, mail server - are called by cisco "internet edge". details: cisco.com/c/en/us/td/docs/solutions/Enterprise/Security/…
    – pyatka
    Apr 24, 2014 at 11:08

1 Answer 1


The layer you labeled in your diagram is considered the core (also called the backbone).

The firewalls in your diagram form a boundary on the core layer, since they transition to another network. Core networks are local to your company or autonomous system.

Keep your core layer as stable as possible. That typically means keeping changes to a minimum, as well as not homing servers or users directly in the core layer. Typically you put your fastest elements in the core; over time the core devices may be repurposed to the edge of the network as the former core devices loose capacity to handle evolving loads.

  • I marked that in the picture as Core, so is that correct? Thank you for details, I really appreciate it - I need to study the boundaries!
    – Pietros
    Apr 24, 2014 at 12:01
  • Based on the information in the diagram, your core boundaries are correct Apr 24, 2014 at 12:03

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