I found the next topology in a cisco course
(layer 2 enviroment)
A router(I think 2811)---Switch(Aggregation switch)---Access Switch
And the course it says "The aggregation switch is required in this
design to aggregate access layer traffic (10/100Mbps) from PCs,
desktops, IP phones and other devices to the router." and Its all.
I dont underestand the pourpose of the aggregation layer,
WHAT IS HIS PURPOSE???
A PC on the 3rd floor connects to the access layer switch in the wiring closet (along with dozens of other devices).
The access layer switch has a trunk port down to the datacenter (ground floor) which terminates at a switch, along with many other connections from other access layer switches on other floors. This switch in the datacenter is the aggregation switch, and is really just part of the distribution layer. If it is a multi-layer switch, it contains the routing function for that switch block. If it doesn't include a L3 function, it could hand off the job to a router via router on a stick or something similar.
I don't believe there is a separate "aggregatin layer" outside of what was mentioned in the previous paragraph.
Without the pretty picture... An aggregation switch is doing what it says... Aggregating.
You will typically have lots of users with multiple switches. So unless you are going to have a router with lots of ports (one per switch?) then you'll aggregate at some point.
Much has to do with speeds, feeds, and bottleneck design/avoidance.
But it's just a term!
The types of layers described are 2 and 3. Layer 2 represents the switching layer. Layer 3 the routing. Layer 1 is the physical connection between devices.
A layer 2 aggregation switch is just a switch that provides connectivity for several other Layer 2 switches. Think of one to many. It's the one switch that many other switches will be connecting to.
At the top the pyramid is Layer 3, the router, which will connect to the aggregation switch.
All I would point out is that for the purposes of the CCDA the 'aggregation layer', whilst synonymous with the 'distribution layer' only pertains to data centre design.
From a Cisco perspective this increasingly employs the use of the Nexus product family with 10GE & 40/100GE being the main drivers.