Internetworking Design Basics 2-3
Switching Overview
Layer 2 addresses are determined by the manufacturer of the data communications equipment used.
They are unique addresses that are derived in two parts: the manufacturing (MFG) code and the
unique identifier. The MFG code is assigned to each vendor by the IEEE. The vendor assigns a
unique identifier to each board it produces. Except for Systems Network Architecture (SNA)
networks, users have little or no control over Layer 2 addressing because Layer 2 addresses are fixed
with a device, whereas Layer 3 addresses can be changed. In addition, Layer 2 addresses assume a
flat address space with universally unique addresses.
Layer 3 switching operates at the network layer. It examines packet information and forwards
packets based on their network-layer destination addresses. Layer 3 switching also supports router
functionality.
For the most part, Layer 3 addresses are determined by the network administrator who installs a
hierarchy on the network. Protocols such as IP, IPX, and AppleTalk use Layer 3 addressing. By
creating Layer 3 addresses, a network administrator creates local areas that act as single addressing
units (similar to streets, cities, states, and countries), and assigns a number to each local entity. If
users move to another building, their end stations will obtain new Layer 3 addresses, but their Layer
2 addresses remain the same.
As routers operate at Layer 3 of the OSI model, they can adhere to and formulate a hierarchical
addressing structure. Therefore, a routed network can tie a logical addressing structure to a physical
infrastructure, for example, through TCP/IP subnets or IPX networks for each segment. Traffic flow
in a switched (flat) network is therefore inherently different from traffic flow in a routed
(hierarchical) network. Hierarchical networks offer more flexible traffic flow than flat networks
because they can use the network hierarchy to determine optimal paths and contain broadcast
domains.
Implications of Layer 2 and Layer 3 Switching
The increasing power of desktop processors and the requirements of client-server and multimedia
applications have driven the need for greater bandwidth in traditional shared-media environments.
These requirements are prompting network designers to replace hubs in wiring closets with
switches.
Although Layer 2 switches use microsegmentation to satisfy the demands for more bandwidth and
increased performance, network designers are now faced with increasing demands for intersubnet
communication. For example, every time a user accesses servers and other resources, which are
located on different subnets, the traffic must go through a Layer 3 device. Figure 2-1 shows the route
of intersubnet traffic with Layer 2 switches and Layer 3 switches.
Figure 2-1 Flow of intersubnet traffic with Layer 2 switches and routers.
As Figure 2-1 shows, for Client X to communicate with Server Y, which is on another subnet, it must
traverse through the following route: first through Switch A (a Layer 2 switch) and then through
Router A (a Layer 3 switch) and finally through Switch B (a Layer 2 switch). Potentially there is a
tremendous bottleneck, which can threaten network performance, because the intersubnet traffic
must pass from one network to another.
Router A
Layer 3 switch
Switch A
Layer 2 switch
Client X
Subnet 1
Switch B
Layer 2 switch
Server Y
Subnet 2
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