Specifications

Copyright © 2009, Juniper Networks, Inc. 15
DESIGN GUIDE - Data Center LAN Connectivity Design Guide
Access Layer Design Considerations
Application and Server Architectures
Another way to look at the access requirements of the data center is via the common three-tier application model
upon which a majority of Web-based applications are built. It defines application architectures in the following
modular components:
Web1.
Application2.
Database3.
Figure 6: The three-tier application model
Today, most Web-based applications are built upon this model. This model runs separate processes on the same
machine or across different networked servers. While Web servers and application servers may share the same
machine or set of servers, it is common to separate the database on a separate machine or set of servers dedicated
to that task.
Benefits and Challenges of the Three-Tier Model
When server farms are used, this model provides built-in HA because any individual server can be taken out of
service without disrupting service since the same function runs on another server belonging to the same application
tier. In that same manner, additional machines can be added to seamlessly scale capacity as needed. Load balancing
the traffic between tiers improves performance and HA. Security is built in as attacks on one server are insulated
from others. For example, a hacked Web server compromises only that server without gaining access to the
application or database servers. Security can be further enhanced by placing firewalls between tiers of servers or
virtualizing a high-end firewall to inspect traffic between the layers and enforce security policies. VLANs can also
increase security by segmenting traffic and reduce the server farm complexity. For increased performance and
security, physical segregation may be desired.
There are a few disadvantages to the three-tier application model. This model does not work as well as other
topologies for computational-intensive applications such as financial modeling, animation, manufacturing and
search engines. Another disadvantage is that often complex traffic engineering is required to optimize performance.
Finally, the TCO of this architecture can be high due to inefficient use of physical server infrastructure with high
power, cooling and space requirements.
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Data
Apps
Web