R&M Data Center Handbook

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3.3. Network Hierarchy
A hierarchical network design subdivides the network into discrete layers. Each of these layers provides specific
functions that define its role within the overall network. When the different functions provided in a network are
made separate, the network design becomes modular and this also results in optimal scalability and performance.
As compared to other network designs, a hierarchical network is easier to administrate and to expand, and problems
can be solved more quickly.
3.3.1. Three Tier Network
Three Tier Networks consist of an Access Layer with switches to desktops, servers and storage resources; an
Aggregation/Distribution Layer in which switching centers combine and protect (e.g. via firewalls) the data streams
forwarded from the Access Layer, and the Core Switch Layer, which regulates the traffic in the backbone.
Core
Aggregation
Access
Storage
FO
Copper
Backbone
LAN
Distribution
Switches
SAN
Switches
LAN
Switches
Storage
Servers
Three Tier Networks resulted from
Two Tier Networks, toward the
end of the 1990s. These Two Tier
networks were pushing their capacity
limits. The bottleneck that was
created was able to be rectified by
using an additional aggregation
layer. From a technical point of
view, the addition of a third layer
was therefore a cost-effective,
temporary solution for resolving
the performance problems of that
time.
Both network architectures are
governed by the Spanning Tree
Protocol (STP). This method was
developed in 1985 by Radia
Perlman, and determines how
switching traffic in the network
behaves. However, after 25 years,
the STP is pushing its limits. The
IETF (Internet Engineering Task
Force) therefore intends to
replace STP by the Trill Protocol
(Transparent Inter-connection of
Lots of Links) internalized by the
STP.
Network protocols for redundant
paths are described in more detail
in section 3.8.6.
Trends such as virtualization and
the associated infrastructure
standardization represent a new
challenge for these Three Tier
Networks.
Rampant server virtualization is making for increasing complexity. For example, ten virtual servers can operate on
one physical computer, and can also be moved as needed from hardware to hardware, in a very flexible manner.
So if a network previously had to manage the data traffic of 1,000 servers, this now becomes 10,000 virtual
machines, which are still in motion to make matters worse.
Classic networks built on three tiers, which dominated data centers from the late 90s, have been having more and
more trouble with this complexity. The call is becoming increasingly loud for a flat architecture, in which the
network resembles a fabric of nodes with equal rights. Instead of just point-to-point connections, crossconnections
between nodes are also possible and increase the performance of this type of network.
Page 54 of 156 © 08/2011 Reichle & De-Massari AG R&M Data Center Handbook V2.0