Network Topologies, Architectures, and Types

This chapter is from the book

CompTIA Network+ N10-009 Exam Cram, 8th Edition

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This chapter is from the book

This chapter is from the book 

CompTIA Network+ N10-009 Exam Cram, 8th Edition

Learn More Buy

For more information on the official CompTIA Network+ exam topics, see the “About the Network+ Exam” section in the Introduction.

A variety of physical and logical network layouts are in use today. As a network administrator, you might find yourself working on these different network layouts or topologies. Therefore, you must understand how they are designed to function.

This chapter reviews general network considerations, such as the various topologies used on today’s networks, local-area networks (LANs), and wide-area networks (WANs).

Network Topologies

• 1.6 Compare and contrast network topologies, architectures, and types.

A topology refers to a network’s physical and logical layout. A network’s physical topology refers to the actual layout of the computer cables and other network devices. A network’s logical topology refers to the way in which the network appears to the devices that use it. Network topology diagrams are used to identify network components and how they are physically or logically connected.

Several topologies are in use on networks today. Some of the more common topologies are the star/hub and spoke, mesh, and hybrid models. The following sections provide an overview of each as well as look at some older topologies you may encounter.

Star/Hub and Spoke

In the star topology, all computers and other network devices connect to a central device called a hub or switch and, for that reason, it is also called a hub and spoke network. Each connected device requires a single cable to be connected to the hub or switch, creating a point-to-point connection between the device and the hub or switch.

Using a separate cable to connect to the hub or switch allows the network to be expanded without disruption. A break in any single cable does not cause the entire network to fail. Figure 2.1 shows a star/hub and spoke topology.

FIGURE 2.1 Star/Hub and Spoke Topology

The star/hub and spoke topology is the most widely implemented network design in use today, but it is not without shortcomings. Because all devices connect to a centralized hub or switch, this creates a single point of failure for the network. If the hub or switch fails, any device connected to it cannot access the network. Because of the number of cables required and the need for network devices, the cost of a star/hub and spoke network is often higher than other topologies. Table 2.1 summarizes the advantages and disadvantages of the star/hub and spoke topology.

TABLE 2.1 Advantages and Disadvantages of the Star/Hub and Spoke Topology

Mesh Topology

When it comes to the mesh topology, it is helpful to differentiate between a wired and wireless implementation, so we focus on the former here and the latter in Chapter 6, “Wireless Solutions.” Mesh incorporates a unique network design in which each computer on the network connects to every other, creating a point-to-point connection between every device on the network. Since this is often done physically, the term wired mesh or wired mesh topology is sometimes used. The purpose of the mesh design is to provide a high level of redundancy. If one network cable fails, the data always has an alternative path to get to its destination; each node can act as a relay.

The wiring for a mesh network can be complicated, as illustrated by Figure 2.2. Furthermore, the cabling costs associated with the mesh topology can be high, and troubleshooting a failed cable can be tricky. As a result, the mesh topology is not the first choice for many wired networks but is more popular with servers/routers.

FIGURE 2.2 Mesh Topology

Table 2.2 summarizes the advantages and disadvantages of the mesh topology.

TABLE 2.2 Advantages and Disadvantages of the Mesh Topology

Hybrid Topology

A variation on a true mesh topology is the hybrid or hybrid mesh. It creates a redundant point-to-point network connection between only specific network devices (such as the servers). The hybrid mesh is most often seen in WAN implementations but can be used in any network.

Another way of describing the degree of mesh implementation is by labeling it as either partial or full. If it is a true mesh network with connections between each device, it can be labeled full mesh, and if it is less than that—a hybrid of any sort—it is called a partial mesh network.

Many of the topologies found in large networking environments are a hybrid of physical topologies. An example of a hybrid topology is the star/hub and spoke bus—a combination of the star/hub and spoke topology and the bus topology (explored further later in this chapter). Figure 2.3 shows how this might look in a network implementation.

FIGURE 2.3 A Star/Hub and Spoke Bus Topology

Point to Point

A point to point (often written with hyphens: point-to-point) networking topology is a network configuration where two devices or nodes are directly connected to each other. In this topology, there is a dedicated communication link between the two endpoints, and data flows directly between them without the involvement of any intermediary devices. While the distinction can be nitpicky, mesh and star/hub and spoke networks are used to connect multiple devices to a network, while point-to-point topology is used to connect two devices.

It might not be the ideal choice for larger, more complex networks. In such cases, more scalable and flexible topologies like star/hub and spoke or mesh networks are typically preferred, since they can accommodate multiple devices and provide greater redundancy and fault tolerance.

Spine and Leaf

Tiered models of computer network architecture are a way of organizing and structuring a network infrastructure into distinct layers (or tiers), each with specific functions and responsibilities. These models help simplify network design, management, and scalability.

A two-tier model that Cisco promotes for switches is the spine and leaf model. In this model, the spine is the backbone of the network, just as it would be in a skeleton and is responsible for interconnecting all the leaf switches in a full-mesh topology. Thanks to the mesh, every leaf is connected to every spine, and the path is randomly chosen so that the traffic load is evenly distributed among the top-tier switches. If one of the switches at the top tier were to fail, there would only be a slight degradation in performance throughout the datacenter.

Because of the design of this model, no matter which leaf switch is connected to a server, the traffic always has to cross the same number of devices to get to another server. This keeps latency at a steady level.

When top-of-rack (ToR) switching is incorporated into the network architecture, switches located within the same rack are connected to an in-rack network switch, which is connected to aggregation switches (usually via fiber cabling). The big advantage of this setup is that the switches within each rack can be connected with cheaper copper cabling and the cables to each rack are all that need to be fiber.

Three-Tier Hierarchical Model

Just as the spine and leaf is a two-tiered model, it is possible to improve system performance, as well as to improve security, by implementing an n-tiered model (wherein the n- can be one of several different numbers).

If we were looking at a database, for example, with a one-tier model, or single-tier environment, the database and the application exist on a single system. This is common on desktop systems running a standalone database. Early UNIX implementations also worked in this manner; each user would sign on to a terminal and run a dedicated application that accessed the data. With two-tier architecture, the client workstation or system runs an application that communicates with the database that is running on a different server. This common implementation works well for many applications. With three-tiered architecture, otherwise known as a three-tier hierarchical model, security is enhanced. In this model, the end user is effectively isolated from the database by the introduction of a middle-tier server. This server accepts requests from clients, evaluates them, and then sends them on to the database server for processing. The database server sends the data back to the middle-tier server, which then sends the data to the client system. Becoming common in business today, this approach adds both capability and complexity.

While the examples are of database tiering, this same approach can be taken with devices such as routers, switches, and other servers. In a three-tiered model of routing and switching, the three tiers would be the core, the distribution/aggregation layer, and the access/edge. We walk through each of the layers present in this scenario.

Core Layer

The core layer is the backbone: the place where switching and routing meet (switching ends, routing begins). It provides high-speed, highly redundant forwarding services to move packets between distribution layer devices in different regions of the network. The core switches and routers would be the most powerful in the enterprise (in terms of their raw forwarding power) and would be used to manage the highest-speed connections (such as 100 Gigabit Ethernet). Core switches also incorporate internal firewall capability as part of their features, helping with segmentation and control of traffic moving from one part of the network to another.

Distribution/Aggregation Layer

The distribution layer, or aggregation layer (sometimes called the workgroup layer), is the layer in which management takes place. This is the place where Quality of Service (QoS) policies are managed, filtering is done, and routing takes place. Distribution layer devices can be used to manage individual branch-office WAN connections, and this is considered to be smart (usually offering a larger feature set than switches used at the access/edge layer). Lower latency and larger MAC address table sizes are important features for switches used at this level because they aggregate traffic from thousands of users rather than hundreds (as access/edge switches do).

Access/Edge Layer

Switches that allow end users and servers to connect to the enterprise are called access switches or edge switches, and the layer where they operate in the three-tiered model is known as the access layer, or edge layer. Devices at this layer may or may not provide Layer 3 switching services; the traditional focus is on minimizing the cost of each provisioned Ethernet port (known as “cost-per-port”) and providing high port density. Because the focus is on connecting client nodes, such as workstations to the network, this is sometimes called the desktop layer.

Table 2.3 highlights each of the layers of the three-tier hierarchical model.

TABLE 2.3 Three-tier Hierarchical Model Layers

Collapsed Core

With a collapsed core architecture, the three-tier model becomes a two-tier model as the core and distribution layers are combined. While a three-tier model is necessary for complex installations that require access by multiple sites, devices, and users, the collapsed core approach is commonly used in datacenters and enterprise networks.

By collapsing the core and distribution layers into a single layer, the streamlined design provides both advantages and trade-offs. Advantages include simplicity, cost savings (via a reduced number of network devices and switches), efficient communication (with fewer layers, there are fewer network hops between devices), and scalability (it can scale relatively well for medium-sized networks and datacenters with a moderate number of devices). Disadvantages include limited redundancy (a failure at the collapsed core layer could potentially impact the entire network), scalability constraints (it is not ideal for extremely large networks or datacenters with high traffic demands), and potential bottlenecks (there is a risk of network congestion if not properly designed and managed).

Traffic Flows

Traffic flows within a datacenter typically occur within the framework of one of two models: East-West or North-South. The names may not be the most intuitive, but the East-West traffic model means that data is flowing among devices within a specific datacenter, whereas North-South means that data is flowing into the datacenter (from a system physically outside the datacenter) or out of it (to a system physically outside the datacenter).

The naming convention comes from the way diagrams are drawn: data staying within the datacenter is traditionally drawn on the same horizontal line (East-to-West), while data leaving or entering is typically drawn on a vertical line (North-to-South). With the increase in virtualization being implemented at so many levels, the East-West traffic has increased in recent years. Table 2.4 summarizes the traffic flow possibilities.

TABLE 2.4 Traffic Flow Options

Older Topologies: Bus and Ring

There are two topologies that have been removed from this iteration of the CompTIA Network+ exam objectives that you will very likely encounter in the workplace: bus and ring. For that reason, it is highly suggested that you be aware of them, and coverage of them is included at the end of this chapter rather than in with the exam fodder.

A bus topology uses a trunk or backbone to connect all the computers on the network, as shown in Figure 2.4. Systems connect to this backbone using T connectors or taps (known as a vampire tap, if you must pierce the wire). To avoid signal reflection, a physical bus topology requires that each end of the physical bus be terminated, with one end also being grounded. Note that a hub or switch is not needed in this installation, and loose or missing terminators from a bus network disrupt data transmissions.

FIGURE 2.4 Physical Bus Topology

Table 2.5 summarizes the advantages and disadvantages of the bus topology.

TABLE 2.5 Advantages and Disadvantages of the Bus Topology

The ring topology is a logical ring, meaning that the data travels in a circular fashion from one computer to another on the network. It is not a physical ring topology. Figure 2.5 shows the logical layout of a ring topology. Note that a hub or switch is not needed in this installation either.

FIGURE 2.5 Logical Design of a Ring Topology

In a true ring topology, if a single computer or section of cable fails, the signal is interrupted. The entire network becomes inaccessible. Network disruption can also occur when computers are added to or removed from the network, making it an impractical network design in environments where the network changes often.

Ring networks can be set up in a fault-tolerant design, meaning that they have primary and secondary rings. If one ring fails, data can use the second ring to reach its destination. Naturally, the addition of the second ring adds to the cost of the network as well as the complexity. Table 2.6 summarizes the advantages and disadvantages of the ring topology.

TABLE 2.6 Advantages and Disadvantages of the Ring Topology