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This chapter is from the book

Configuring Routers and Switches

  • Given a scenario, install and configure routers and switches.

The next chapter focuses on actual hardware components of a network, but the reason for the hardware is to carry out the operations discussed in this chapter. This section looks at a few of the more advanced features that routers and switches perform.

Power over Ethernet (PoE)

The purpose of Power over Ethernet (PoE) is pretty much described in its name. Essentially, PoE is a technology that enables electrical power to transmit over twisted-pair Ethernet cable. The power transfers, along with data, to provide power to remote devices. These devices may include remote switches, wireless access points, voice over IP (VoIP) equipment, and more.

One of the key advantages of PoE is the centralized management of power. For instance, without PoE, all remote devices need to be independently powered. In the case of a power outage, each of these devices requires an uninterruptible power supply (UPS) to continue operating. A UPS is a battery pack that enables devices to operate for a period of time. With PoE supplying power, a UPS is required only in the main facility. In addition, centralized power management enables administrators to power up or down remote equipment.

The Spanning Tree Protocol (STP)

An Ethernet network can have only a single active path between devices on a network. When multiple active paths are available, switching loops can occur. Switching loops are simply the result of having more than one path between two switches in a network. Spanning Tree Protocol (STP) is designed to prevent these loops from occurring.

STP is used with network bridges and switches. With the help of Spanning Tree Algorithm (STA), STP avoids or eliminates loops on a Layer 2 bridge.

STA enables a bridge or switch to dynamically work around loops in a network’s topology. Both STA and STP were developed to prevent loops in the network and provide a way to route around any failed network bridge or ports. If the network topology changes, or if a switch port or bridge fails, STA creates a new spanning tree, notifies the other bridges of the problem, and routes around it. STP is the protocol, and STA is the algorithm STP uses to correct loops.

If a particular port has a problem, STP can perform a number of actions, including blocking the port, disabling the port, or forwarding data destined for that port to another port. It does this to ensure that no redundant links or paths are found in the spanning tree and that only a single active path exists between any two network nodes.

STP uses bridge protocol data units (BPDUs) to identify the status of ports and bridges across the network. BPDUs are simple data messages exchanged between switches. BPDUs contain information on ports and provide the status of those ports to other switches. If a BPDU message finds a loop in the network, it is managed by shutting down a particular port or bridge interface.

Redundant paths and potential loops can be avoided within ports in several ways:

  • Blocking: A blocked port accepts BPDU messages but does not forward them.
  • Disabled: The port is offline and does not accept BPDU messages.
  • Forwarding: The port is part of the active spanning tree topology and forwards BPDU messages to other switches.
  • Learning: In a learning state, the port is not part of the active spanning tree topology but can take over if another port fails. Learning ports receive BPDUs and identify changes to the topology when made.
  • Listening: A listening port receives BPDU messages and monitors for changes to the network topology.

Most of the time, ports are in either a forwarding or blocked state. When a disruption to the topology occurs or a bridge or switch fails for some reason, listening and learning states are used.


In computer networking, the term trunking refers to the use of multiple network cables or ports in parallel to increase the link speed beyond the limits of any one cable or port. Sound confusing? If you have network experience, you might have heard the term link aggregation, which is essentially the same thing. It is just using multiple cables to increase the throughput. The higher-capacity trunking link is used to connect switches to form larger networks.

VLAN trunking—or VLAN (trunking), as CompTIA lists it—is the application of trunking to the virtual LAN—now common with routers, firewalls, VMWare hosts, and wireless access points. VLAN trunking provides a simple and cheap way to offer a nearly unlimited number of virtual network connections. The requirements are only that the switch, the network adapter, and the OS drivers all support VLANs. The VLAN Trunking Protocol (VTP) is a proprietary protocol from Cisco for just such a purpose.

Port Mirroring

You need some way to monitor network traffic and monitor how well a switch works. This is the function of port mirroring. To use port mirroring, administrators configure a copy of all inbound and outbound traffic to go to a certain port. A protocol analyzer examines the data sent to the port and therefore does not interrupt the flow of regular traffic.

Port Authentication

Port authentication is what it sounds like—authenticating users on a port-by-port basis. One standard that specifies port authentication is the 802.1X standard, often associated with wireless security. Systems that attempt to connect to a LAN port must be authenticated. Those who are authenticated can access the LAN; those who are not authenticated get no further. Chapter 10 provides more information on the 802.1X standard and port authentication.

Cram Quiz

  1. Port mirroring enables administrators to monitor which traffic to the switch?

    • circle.jpg A. Inbound only
    • circle.jpg B. Outbound only
    • circle.jpg C. Inbound and outbound
    • circle.jpg D. Neither inbound nor outbound
  2. Which of the following is NOT used to avoid redundant paths and potential loops within ports?
    • circle.jpg A. Blocking
    • circle.jpg B. Learning
    • circle.jpg C. Forwarding
    • circle.jpg D. Jamming

Cram Quiz Answers

  1. C. Port mirroring enables administrators to monitor the traffic outbound and inbound to the switch.
  2. D. The common methods to avoid redundant paths and potential loops within ports include blocking, disabled, forwarding, learning, and listening. Jamming is not one of the methods employed.
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