Network Topologies

You already have been introduced to several different network topologies as you have read through this book. For instance, 10BASE2 networks use a physical bus topology, whereas 10BASE-T networks use a physical star topology. This section introduces you to several other types of network topologies.

Figure 11-1 shows the different types of Ethernet topologies covered earlier in the book, with some specific terms used to describe the topology for each design.

Figure 11-1 Different Types of Network Topologies for Ethernet So Far in This Book

Physical Bus Logical Bus

Hub1

Hub1

rrrrrrrrr

Switch 1

Physical Star Logical Bus

Physical Star Logical Star

Switch 1

The figure shows a 10BASE5 network, a 10BASE-T network using a shared hub, and a switch with 10/100 links. Physically, the topologies with the hub and the switch look a little like how a child might draw a star, or the sun, with a center (the hub or switch) and with beams of light pointing outward (like the Ethernet cables to the PCs in the figure). Star topologies also are called hub-and-spoke topologies.

Physical bus topologies transmit the electrical signal from one end of a cable to the other, with the signal being picked up at each connection point.

The term logical topology refers to how the network behaves. For instance, from Chapter 3, you know that a 10BASE-T hub repeats an incoming signal out every other port on the hub.

So, logically, it also causes the electrical signals to be sent to every connection on the network—more like a bus in logic. So, people might describe a network using a hub as a physical star, but a logical bus. The logical topology for the switch network is a star because, unlike a hub, a switch does not repeat the signal out every port, but just to the appropriate device.

Figure 11-2 shows three other types of network topologies, which could be used for interconnecting Ethernet hubs and switches.

Figure 11-2 Extended Star, Full Mesh, and Partial Mesh

Figure 11-2 Extended Star, Full Mesh, and Partial Mesh

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The extended star is characterized by parts of the topology that look like a star, which, in turn, are connected in star fashion to some other node. For instance, the three switches with PC attached, taken alone, form a star topology. By connecting to another switch in the middle, another star is formed, so this topology would be characterized as an extended star. Extended star topologies are rare for Ethernets.

If you pursue your CCDA certification, you will come across many designs in which you see the full mesh and partial mesh topologies shown in the figure. A full mesh is typical of switches that collectively form the core and distribution layers of a campus LAN design that includes Layer 3 switching. The partial mesh design often is found between distribution layer and access layer switches. If you want to learn more about LAN design concepts, pick up the CCDA Exam Certification Guide and read more. For our purposes, you should just know that a full mesh means that all the respective nodes in the network have a direct connection. A partial mesh means that some of the nodes in a network have a direct connection, but others do not.

Frame Relay networks often are described as being full mesh or partial mesh. For instance, consider Figure 11-3, with a Frame Relay network.

Figure 11-3 Physical Star, with Full and Partial Mesh

Figure 11-3 Physical Star, with Full and Partial Mesh

Frame Relay networks use a physical star topology because each Frame Relay DTE device connects to a central Frame Relay network. However, depending on which sites have VCs connecting them, the logical design is either full mesh or partial mesh. When you study for the ICND exam, you will see many Frame Relay examples with full- and partial-mesh designs. Practically, no one really refers to Frame Relay as using a physical star design, but many people do refer to a Frame Relay design as either full mesh or partial mesh.

Figure 11-4 shows the final topology covered here—the ring topology. Figure 11-4 Single-Ring and Dual-Ring Topologies a,

Figure 11-4 shows the final topology covered here—the ring topology. Figure 11-4 Single-Ring and Dual-Ring Topologies

Atlas Track Curve Dimensions

Physical Single Ring Logical Ring

Physical Dual Ring Logical Ring

* Token Ring Hub

Physical Single Ring Logical Ring

Physical Dual Ring Logical Ring

Physical Star Logical Ring

The left-most figure depicts a concept in which each device is cabled to the next, with the signal transmitted in a single direction. By doing so, the signal starts with one device, and eventually, the signal makes it all the way back to the original sender of the data. With one physical path, the topology is called a single ring, and with two physical paths, the topology is called a dual ring.

Dual rings are useful for failover. With dual rings, one ring is used to transmit data under normal operations, with the second ring for failover. If the physical path between two adjacent devices fails, the two devices on either side of the problem simply can loop the signal on one ring onto the other—and another physical loop has been created. This dual-ring topology was used with the now outdated Token Ring and Fiber Distributed Data Interface (FDDI) LANs. The same concept is used in optical networking today.

Finally, on the right side of Figure 11-4, you see an example of how Token Ring was cabled— back when anyone cared. Each device had a cable connecting it to a hub, with a transmit wire and a receive wire inside the cable. The electrical signal was sent down each wire and was repeated back up the wire to the hub; then the hub repeated the process with the next device, and so on. Electrically, a single ring was created, so Token Ring uses a logical ring topology, but physically, it uses a star topology.

Table 11-2 summarizes the types of physical topology covered in this chapter. Table 11-2 Physical Topology Types

Term

Definition

Bus

This is a linear topology, with all devices connected to the cable.

Star

Each device is connected to a central point. Sometimes called hub-and-spoke.

Extended star

A star topology is in the center, but instead of each point being a single device, it can be the center of another star topology.

Full mesh

Each device has a direct connection to each other device.

Partial mesh

Each device does not have a direct connection to each other device.

Single ring

Each device is connected directly to two others so that the signal is repeated in one direction, creating a ring or loop.

Dual ring

Two rings go through the same set of devices, allowing loops to be made upon failure, which continues the operation of a ring.

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