R&M Data Center Handbook

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3.3.2. Access Layer
The Access Layer creates the connection to terminal devices such as PCs, printers and IP phones, and arranges
access to the remainder of the network. This layer can include routers, switches, bridges, hubs and wireless
access points for wireless LAN. The primary purpose of the Access Layer is to provide a mechanism which
controls how devices are connected to the network and which devices are allowed to communicate in the network.
For this reason, LAN switches in this layer must support features like port security, VLANs, Power over Ethernet
(PoE) and other necessary functionalities. Switches in this layer are often also known as Edge Switches.
Multiple access switches can often be stacked with one another into one virtual switch, which can then be
managed and configured through one master device, since the stack is managed as one object.
Manufacturers sometimes also offer complete sets of security features for connection and access control. One
example of this is a user authentication system at the switch port in accordance with IEEE802.1x, so only
authorized users have access to the network, while malicious data traffic is prevented from spreading.
New Low Latency Switches specifically address the need for I/O consolidation in the Access Layer, for example in
densely packed server racks. Cabling and management are simplified dramatically and power consumption and
running costs drop while availability increases. These switches already support Fibre Channel over Ethernet
(FCoE) and thus the construction of a Unified Fabric. Unified Fabric combines server and storage networks into
one common platform that can be uniformly administered, which paves the way for extensive virtualization of all
services and resources in the data center.
Product example from Cisco
3.3.3. Aggregation / Distribution Layer
The Aggregation Layer, also known as the Distribution Layer, combines the data that was received from the
switches in the Access Layer before they are forwarded on to the Core Layer to be routed to the final receiver.
This layer controls the flow of network data with the help of guidelines and establishes the broadcast domains by
carrying out the routing functions between VLANs (virtual LANs) that are defined in the Access Layer. This routing
typically occurs in this layer, since Aggregation Switches have higher processing capacities than switches in the
Access Layer. Aggregation Switches therefore relieve Core Switches of the need to carry out this routing function.
Core Switches are already used to capacity for forwarding on very large volumes of data. The data on a switch
can be subdivided into separate sub-networks through the use of VLANs. For example, data in a university can be
subdivided by addressee into data for individual departments, for students and for visitors.
In addition, Aggregation Switches also support ACLs (Access Control Lists) which are able to control how data are
transported through the network. An ACL allows the switch to reject specific data types and to accept others. This
way they can control which network devices may communicate in the network. The use of ACLs is processorintensive,
since the switch must examine each individual packet to determine whether it corresponds to one of the
ACL rules defined for the switch. This examination is carried out in the Aggregation Layer, since the switches in
that layer generally have processing capacities that can manage the additional load. Aggregation Switches are
usually high-performance devices that offer high availability and redundancy in order to ensure network reliability.
Besides, the use of ACLs in this layer is comparatively simple. So instead of using ACLs on every Access Switch
in the network, they are configured on Aggregation Switches, of which there are fewer, making ACL administration
significantly easier.
Combining lines contributes to the avoidance of bottlenecks, which is high importantly in this layer. If multiple
switch ports are combined, data throughput can then be multiplied by their number (e.g. 8 x 10 Gbit/s = 80 Gbit/s).
The term Link Aggregation is used in accordance with IEEE 802.1ad to describe switch ports that are combined
(parallel switching), a process called EtherChannel by Cisco and also trunking by other providers.
Due to the functions they provide, Aggregation Switches are severely loaded by the network. It is important to
emphasize that these switches support redundancy, for purposes of their availability. The loss of one Aggregation
Switch can have significant repercussions on the rest of the network, since all data from the Access Layer are
forwarded to these switches. Aggregation Switches are therefore generally implemented in pairs so as to ensure
their availability. In addition, it is recommended that switches be used here that support redundant network
components that can be replaced during continuous operation.
R&M Data Center Handbook V2.0 © 08/2011 Reichle & De-Massari AG Page 55 of 156