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What Is Network Topology?

Network topology is used to describe the physical and logical structure of a network. It maps the way different nodes on a network--including switches and routers--are placed and interconnected, as well as how data flows. Diagramming the locations of endpoints and service requirements helps determine the best placement for each node to optimize traffic flows.

What does a network topology do?

A well-planned network topology enhances the user experience and helps administrators maximize performance while fulfilling business needs. When the right topology is chosen for a business's needs, it is easier to locate faults, troubleshoot and fix problems, and share resources across networks.

With a properly managed network topology, a business can improve the efficiency of its data transfer. Better efficiency, in turn, helps reduce costs for maintenance and operations.

What kinds of network topologies are there?

Network topologies are generally arranged in two ways:

Physical (or underlay): This maps the actual connections in a network, such as wires and cables and the placement of various components.

Logical (or overlay): This shows how data flows within a network and from one device to another, regardless of the physical connections among devices. While the logical network uses the physical connections for data transfer, the actual flow of data is defined by the logic not the physical connections.

How do I diagram a network topology?

Network topologies can be sketched out on paper, but it is easier to use software programs that are purpose-built for network diagramming. The programs usually have pre-built templates, as well as symbols for common network elements like routers and switches.  

By diagramming the network, you can see how information moves across the network--and you can identify (and avoid) possible bottlenecks for data traffic. A diagram provides a useful reference point when troubleshooting problems. A topology diagram also offers a clearer picture of network functionality. It can help network administrators identify where new nodes are needed and which ones should be monitored, upgraded, or replaced. 

What needs to be considered when designing network topologies?

IT leaders have to gauge whether a chosen topology is suitable for the network's purpose, such as the size and scale needed to meet business goals as well as the available budget. Other considerations are performance needs and redundancy requirements.

Scalability is also key. If the company is growing, and there are plans to expand the size or complexity of the network, organizations should consider choosing a network topology that can be more easily modified.

What are the limitations of physical topologies?

Physical topologies are difficult to change as organizational needs change. For this reason, physical networks are not as agile as logical networks, and cannot be as easily reconfigured when users increase or devices are added. In addition, the networks' dependency on physical connections places more demands on security teams, which have to rely on firewalls and switch configurations to protect networks.

Logical topologies, on the other hand, are crafted by defining, using, and enforcing fields in packet headers, and therefore can easily be changed to meet changing requirements--as long as the physical underlay has the capacity and scalability to support the needs of the logical topology. 

Physical network topologies

Physical network topologies use to play a much more significant role in network design. Today, as long as the physical underlay is robust and scalable, the emphasis is on designing logical or virtual topologies. The most commonly used physical topologies are described below.

Layered architecture

Commonly used in the enterprise campus and branches, layered architecture--typically comprised of three layers--is a widely adopted model for designing a reliable, scalable, and cost-efficient network that serves users who directly connect to it. The layers are:

  • Access layer: provides workgroup and user access to the network
  • Distribution layer: provides policy-based connectivity and controls the boundary between the access and core layers
  • Core layer: provides fast transport between distribution switches within the enterprise
    In smaller networks, not all layers may be physically present.

Spine-and-leaf architecture

In this two-layer architecture, commonly found in data centers, every lower-tier switch (leaf layer) is connected to each of the top-tier switches (spine layer) in a full-mesh topology. The leaf layer consists of access switches that connect to devices such as servers. The spine layer is the backbone of the network and is responsible for interconnecting all leaf switches.