When you design a
data center using a modular approach, the network is divided into three
functional layers: Core, Aggregation, and Access. These layers can be physical
or logical, and you can add and remove them without redesigning the entire data
center network.
Because of the
hierarchical topology of a modular design, the addressing is also simplified
within the data center network. Modularity implies isolation of building
blocks, which are separated from each other and communicate through specific
network connections between the blocks. Modular design provides easy control of
traffic flow and improved security. In other words, these blocks are
independent from each other; a change in one block does not affect other
blocks. Modularity also enables faster moves, adds, and changes (MACs) and
incremental changes in the network.
Modular networks are
scalable. Scalability allows for the considerable growth or reduction in the
size of a network without making drastic changes. Scalable data center network
design is achieved by using the principle of hierarchy and modularity.
Keep a network as
simple as possible. Modular designs are simple to design, configure, and
troubleshoot.
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Access Layer—The access
layer is the first point of entry into the network for edge devices, end
stations, and servers. The Access layer grants user access to network devices
and provides connectivity to servers. The switches in the access layer are
connected to two separate distribution layer switches for redundancy. The data
center access layer provides Layer 2, Layer 3, and mainframe connectivity. The
design of the access layer varies, depending on whether you use Layer 2 or
Layer 3 access. The access layer in the data center is typically built at Layer
2, which allows better sharing of service devices across multiple servers. This
design also enables the use of Layer 2 clustering, which requires the servers
to be Layer 2 adjacent. With Layer 2 access, the default gateway, you can
configure for the servers at the aggregation layer.
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Aggregation Layer—The
aggregation (or distribution) layer aggregates the uplinks from the access
layer to the data center core. This layer is the critical point for control and
application services. Security and application service devices (such as
load-balancing devices, SSL offloading devices, firewalls, and IPS devices) are
often deployed as modules in the aggregation layer. The aggregation layer
provides policy-based connectivity.
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Core Layer—Also known as
backbone, the core layer provides high-speed packet switching, scalability and
high availability, and fast convergence. Implementing a data center core is a
best practice for large data centers. When you implement the core in an initial
data center design, it eases network expansion and avoids disruption to the
data center environment.
Use the following
criteria to determine whether a core solution is appropriate: The data center
typically connects to the campus core using Layer 3 links. The data center
network is summarized, and the core injects a default route into the data
center network.