Hi,
It depends on your network requirement, budget and business goals.
In general data center's 3 Tier design (Core/Agg/Access), L3 connection is recommended between CORE and Agg layers.
Below is typical 3 Tier Design requirement.
-Core Layer:
•Modular Chassis
•High 10GE Density
•Low Oversubscription
•Layer 3 Links to Aggregation
•Hand-off to Campus
- Aggregation Layer:
•Modular Chassis
•High 10GE Density
•Layer 2 Trunks to Access
•Default Gateway for Servers (L2/L3 boundary)
•Typically Attaches Services
- Access Layer:
•Top of Rack (ToR)
•End/Middle of Row (EoR/MoR)
•GE or 10GE to Servers
•Layer 2 Switching
Layer | Switch Type | Port Speed | Config | Other |
Core | Modular | 10GE | Layer 3 | Campus hand off |
Aggregation | Modular | 10GE | Layer 2/3 boundary | Services (optional) |
Access | Fixed or Modular | GE/10GE | Layer 2 | ToR or MoR |
Also, you can use 2 Tier design by combing Core & Agg.
Cisco usually recommend 3 Tier design if you want to have a building block kind of approach to design as you need something to interconnect all those building blocks.
If one building block is enough to satisfy the needs of the network, then you can collapse essentially the core and aggregation into a single layer
You can see more detail from CISCO Datacenter 3.0 IP Infra document.
Hope this helps.
KK...
