Difference between Access, Distribution and Core layers?

Unanswered Question
Oct 5th, 2007

Hello everyone,

I have come across terms such as Access, Distribution and Core layers. Can someone pls explain me the difference between these?

Thanks in advance.

Satish

I have this problem too.
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sgregg Tue, 10/09/2007 - 09:51

That was a good article to read. Dont feel bad, I had someone working for me once, a CCNA and he just did not understand nor could he with any amount of effort comprehend the 3t tiers.

Just to apply what that document states. Typically in your core of your switched network you will have a couple of big bad boys like some 6500's which are designed to pass massive traffic. You will probably have this at your core datacenter where you might even have direct connectivity to your servers or at minimum your server farm switches. You would not likely have any users or user switches hanging off of these switches. From your core switches you would have some major trunk lines that would extend to major front end closets or distribution points and you would not have any users connected to that switch either, this may be something like a 4500 series switch. Typically the bulk of your STP redundancy is happening at the distribution layer in your infrastructure. Next you would have slews of user access switches going to the various closets across the facility that all tie into the distribution switches. Those user access switches may be something like 3550, 3560, maybe even old 2900 series switches. All of your users workstations would then connect to the user switch. Many times the user switches are daisy chained so the connection goes from core, distro, access-access-access, workstation. You should try to stay under 3 access switches of daisy chain but I have seen many places daisy chain up to 7 or 8 switches, not ideal.

So to recap, core, in your data center tying in all of your switched fabric, providing fast access to your server farm and forwarding out to a router for external networks or Internet. Distribution is running from your core and maybe serves as the central point for an entire building of which then user switches run from the distribution layer to closets on the various floors.

The platforms would greatly vary dependent on the size of infrastructure you are using. Many small infrastructures use a collapsed layer model that combines the core and distro layers. I have also seen very large companies use a collapsed core design with a focus on switching of which you will find great debate on which method is better. I prefer the 3 layer model identified here.

-Shawn

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