In this section you will learn about network hardware devices such as hubs, switches and routers.
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The Network Interface Card (hereafter abbreviated as NIC) is the fundamental component needed to connect a device to a network. All devices from desktop computers to cell phones need some kind of
NIC. NICs can be wired (as in the illustration) or wireless. NICs operation at both the Physical and Data
Link layers of the OSI model. At the Physical level, the NIC contains the transceiver electronics which create and receive signals according to the standard used on the network. The signals represent bits of information necessary to communicate across the network system. NICs are also responsible for organizing bits into frames. The NIC uses the Data Link layer to organize the bits into frames. A frame contains the data along with the physical or MAC address of the sending and receiving NIC control information. The format and number of bits in a frame depend on the networking standard. As shown in the following diagram, in an Ethernet network, each frame consists of from 46 – 1500 bits plus 18 bits of control information.
There are two major LED indicators on the back of a NIC. The Link or LNK indicator (normally green) indicates the NIC has a connection to the network hub or switch. If the indicator is not lite, check the cable and all connections to be sure they are working. The Activity or ACT indicator blinks when frames of data are transmitted or received. If this indicator is not working, check the software driver and verify all network connections. If the indicator is on continuously, the card may be experience heavy network traffic or is jamming the network in a process called jabbering in which case you may need to replace the NIC.

Physical layer devices operate at the bit level and are responsible for encoding and decoding signals that represent 1 and 0 on the network cable.
Resistance and electrical noise in a cable limit the distance a signal can be sent. Repeaters operate at the physical layer of the OSI model and are used to connect network cable segments.
Repeaters allow digital signals to be transmitted over longer distances. A repeater takes digital signals from one port and regenerates the signal on the second port. In addition to extending the distance, repeaters can be used to connect different cable types. For example, a twisted pair cable running 100Base-T can be connected to a 100Base-FX optical fiber cable using a repeater.
A hub is a connection device that allows a number of cables to come together at a central point.
Hubs are actually multi-port repeaters and operate at the physical layer of the OSI model.
Signals coming in one port of the hub are repeated out the other ports. Networks that use hubs are usually referred to as Star networks.
Hubs can be classified as simple or intelligent. Simple or passive hubs simply repeat signals on all ports. Intelligent or managed hubs provide internal processing capabilities the permit remote management, filtering of data, and providing diagnostic information.
Data link layer devices utilize software along with hardware to process groups of bits called frames. Each frame contains the message data along with destination and source addresses that identify the device that is sending the frame and the device that is to receive the frame. A Data Link layer device is capable of recognizing the destination and source addresses and use them to process the data frame. A NIC operates at both the Physical and Data Link layers since it is able to send and receive bits as well as recognize the destination address field and process frames that are addressed to that device.
A Bridge is used to separate networks into two or more collision domains. The bridge listens to traffic on each port in order to learn the MAC addresses used on each of the domains. When a device in one domain sends a frame with the MAC address of a device on another domain, the bridge will transfer the frame to the destination network domain.

A switch is a programmable multi-port bridge that allows each device to be on a separate segment – eliminating the problems of collisions and packet sniffing (the process of a computer reading frames of data intended for another device). Today, low cost switches have largely replaced hubs on most Ethernet networks.
As described on pages 262 – 263, switches may use one of two different methods for transferring data between ports.
As a frame enters the switch, the switch reads the destination MAC address and then determines the destination port . Once the destination port is determined, the remaining bits of the frame are directed out that port as they are received. The CRC is not used to check the frame integrity. This mode is the fastest, but has the following disadvantages:
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Propagates damaged frames which can slow communications across WANs.
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All ports must be at the same speed. The switch cannot have one 1000Base-T port and the other ports running at 100Base-T.
A store-and-forward mode switch reads an entire frame, uses the CRC to check for valid data, and then sends the frame out the appropriate port based on the destination MAC address. While a store-and-forward switch has slightly more overhead, it can provide for better throughput across marginal networks because damaged fames are detected in route. It also has the advantage of sending frames across ports of differing speeds. For example, a store-and-forward switch could have one 10GBase-T port attached to a server and several 1000Base-T ports that are attached to workstations. A frame received from a workstation on the 1000Base-T port could be sent to the server via the 10Gbase-
T port providing faster access time when multiple workstations are accessing the server.
By definition, a broadcast domain is a combination of ports that make up a Layer 2 segment.
Broadcast domains are a critical part of TCP/IP networks since it is through broadcast domains that devices learn the MAC address of other systems. Higher layer protocols work with devices by name or IP-Address. In order for packets to be sent across a LAN, the frames need to have the MAC address of the receiving device. A device learns the MAC address of other devices by using broadcasts. A broadcast is a frame that has all 1’s in its MAC address. All devices that are

part of a network segment (or broadcast domain) receive and process the broadcast frame. The device whose IP-Address is being queried responds by transmitting its MAC address back to the inquiring system. This process is done using the Address Resolution Protocol (ARP). Once a device learns the MAC address of the destination, it saves the mapping of the IP-Address and associated MAC address in a ARP Table so it will not have to go through the broadcast process each time it transmits.
Broadcast domains can be connected together using a layer 3 device such as a router (or Layer 3 switch). We will discuss routers later in this section.
By default, all ports on a switch belong to the same broadcast domain. When multiple switches are linked together, by default all ports on all the switches belong to the same broadcast domain.
Switches can be programmed to group ports into logically separate broadcast domains called
VLANs. Each VLAN is s separate broadcast domain. Devices that are port of a VLAN (broadcast domain) can send frames directly to each other using the ARP protocol – no router.
Read pages 264 – 265 to learn more about VLANs and broadcast domains.

Trunking is the process of allowing multiple VLANs to be connected across a single port. Read pages 265-267 to learn more about trunking.
Notice that trunking requires a special tag field to be inserted into the frame in order to sort out
VLAN traffic when it is received on a switch.
We will discuss more about VLANs and Trunking in or live presentation.

Rather than using routers, today many larger networks are created by forming VLANs by connecting multiple switches. To make the network more reliable, a partial Mesh network can be created using multiple switches as shown below:
The possible problem with Mesh networks that contain multiple switches is that unwanted broadcast loops can be accidentally created. Since a broadcast frame contains all 1s in its destination address, it can be passed around and around the network. Spanning Tree Protocol (STP) is a method to prevent traffic or broadcast loops.
On pages 267-269, the book describes how STP (IEEE 802.1D) is used to block traffic loops. STP (802.1D) was introduced back in the 1980s and is relatively slow for today’s high speed networks. Since that time, a newer version of STP, called Rapid Spanning Tree Protocol (RSTP), was introduced using IEEE 802.1w standard. Today, most network switches are designed to use RSTP with the IEEE 802.1w standard. We will discuss RSTP in more detail during our live presentation.

Network layer or Layer-3 devices are designed to work at the Network layer of the OSI model to move packets (called Datagrams) between logical networks or VLANs.
Originally switches were built to work at the Data Link layer, using the MAC addresses to direct frames of data to the necessary ports. Since the advent of VLANs, switches have had the additional task of using
IP addresses to move packets between broadcast domains. When a computer in a broadcast domain transmits a packet, the switch send the frame to all devices on the Network layer uses the IP address to determine if the packet is going to a device on the same broadcast domain. If the destination is on the same broadcast domain, the Network layer simply uses the ARP process to find the MAC address of the destination device and the Data Link layer send out the frame. If the destination computer is on a different broadcast domain, it will have a different network address. In that case, the Network layer using the IP address to direct the packet to the correct network or VLAN.

Routers are intended to use the network address to direct a packet (called a datagram) to the appropriate network. Each router that a datagram crosses is called a hop. To prevent a datagram from looping around the network, each router changes the datagram’s hop number. When the hop number reaches its limit, the datagram will discarded.
Routers can be classified into three types; Interior, Exterior, and Border routers.
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Interior routers are managed by the organization and are used to route datagram packets between networks within a LAN or MAN.
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Exterior routers are used on the Internet to direct datagram packets between different corporate or organizational networks. Exterior routers use special protocols and are configured by ISPs under the control of the Internet committees.
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Border routers are used to connect a LAN to the Internet backbone. The wireless access point
(WAP) in your home is a simple example of a border router.
In order to select the next hop in the destination path, routers use a Router Table. A Router Table stores
IP Address and other information to select the next hop in the path. Part of the information stored in the router table is used to locate the most efficient way to reach the destination host. Router tables are created an maintained using either Static or Dynamic routing.
In Static routing, the network administrator enters and updates the information in the router table.
While this works for very small networks, it is not feasible to do on larger networks.
In Dynamic routing, a protocol is used to allow routers to create and maintain entries in the router tables.
Read pages 270-273 to learn more about router functions.
Routing protocols are used to maintain entries in the router table. As mentioned earlier, these entries are used by the router to identify the possible paths a datagram can be sent to reach its destination.
Each entry includes the IP Address of the destination network, the next router in the path, and a vector which specifies the cost in terms of time and reliability. Routing protocols can be divided into 3 categories based on how they work; Distance Vector, Link-State (IS-IS), and EIGRP (propriety CISCO protocol). Network + objectives require that you know the major features of these routing protocols and some of the advantages/disadvantages of each.
Read pages 274 – 275 to learn about the various routing protocols and their uses. Following is a summary of the routing protocols based on Network + objectives.

Determine best path based on distance to the destination expressed in either the number of hops or in some cases the latency to reach the destination expressed as a metric.
Periodically exchange router table information with neighboring routers. Requires minimal configuration.
An interior gateway protocol (IGP) is a routing protocol that is used to exchange routing information within an autonomous system (AS). An autonomous system is a private network or
LAN.
The simplest and least efficient protocol.
Used for small internal networks
Maximum of 15 hops
Uses broadcast traffic to send router table information every 30 seconds
RIPv2 generates less broadcast traffic and is more secure
Cisco proprietary Distance Vector protocol.
Replaced the older Exterior Gateway Protocol (EGP).
Used on Internet routers to connect autonomous systems across a public network. Even though most Internet users do not use it directly, BGP is one of the most important protocols of the Internet.
Most Internet service providers must use BGP to establish routing between
Autonomous Systems (AS). An AS-Path is a connection between two independent LANs.
BGP does not involve traditional Interior Gateway Protocol (IGP) metrics, but routing decisions are made based on path, network policies and/or rule-sets.
For this reason, it is more appropriately termed a reachability protocol rather than routing protocol.

The link-state routing protocols enable routers to share information across the network. This information is used by each router to build its own router table. While Distance-vector router table entries are passed from one router to another (built by rumor), link-state router entries are mapped by each router individually using the shared information. There are two major Link-
State protocols; OSPF and IS-IS.
OSPF is used on Interior and Border routers and can coexist with RIP and RIPv2.
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OSPF imposes no hop count limit and uses a more complex algorithm for determining the best path to a destination network.
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While OSPF is more difficult to configure than RIP, it uses less bandwidth and faster convergence time.
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OSPF is supported by all modern routers and is commonly used on LANs and
MANs.
Unlike OSPF, IS-IS (Intermediate System to Intermediate System) is used on interior routers only. While OSPF is TCP/IP dependent, IS-IS is designed to support 2 layer 3 protocols. IS-IS is much less common than OSPF.

EIGRP is based on the older Interior Gateway Procol (IGP) and has characteristics of both
Link-State and Distance-Vector. EIGRP is a proprietary protocol that was developed by
Cisco in the 1980s and may be used on both Interior and Border routers. Like IS-IS, it can support multiple Network layer protocols, but is only supported by Cisco routers.
A WAP is a wireless switch that allows multiple devices to be connected using radio frequencies in addition to physical wires. WAPs typically function as both a switch and router. They work as a switch in that the move data frames between multiple wired and wireless devices using the
MAC address. They also function as a router in that they can move packets from the local network to an Internet connection using the IP address. Many WAPs also work as Transport layer firewall, blocking or allowing traffic onto the local network based on Port numbers. You will learn more about the function and configuration of WAPs and Firewalls in a later sections.

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If you purchase a new desktop computer today, what type of expansion board NIC is it most likely to contain?
Page 240
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If you purchase a new laptop today, what type of NIC is it most likely to have?
Page 246
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Which two of the following IRQs could you probably assign to a NIC without causing a conflict with preassigned devices?
Page 252
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A certain computer on your Fast Ethernet network seems to be acting sluggish. After ensuring the 10/100 Mbps NIC is not malfunctioning, you decide to find out whether it’s configured to transfer data at 10 Mbps rather than 100 Mbps, as it should be. How could you find this information and change it, if necessary?
Page 254
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Suppose computers on your home office network are connected to a single hub (A), but now you need to expand the network. You purchase another hub (B). Assuming you use a straight-through (not a crossover) cable, what port on hub A will you use to connect it to hub B?
Page 256
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You and a friend decide to set up Fast Ethernet networks in your respective houses to connect a half-dozen computers. Both of you will connect your networks to high-speed
Internet connections. As the connectivity device for end nodes, you purchase a router, whereas your friend purchases a 12-port hub. Which of the following will your network do that your friend’s network won’t do?
Page 270-

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You are a network technician working on a 100Base-T network. A coworker has been having trouble logging on to the server and asks whether you can quickly tell her if her workstation’s NIC is operating properly. You do not have the NIC’s utility disk on hand, but you look at the back of her workstation and learn that although the NIC is properly installed and connected to the network, something’s wrong with it. What might you have seen that causes you to come to this conclusion?
Page 251
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How do bridges keep track of whether they should forward or filter frames?
Page 258
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Which of the following is an advantage of using switches rather than hubs?
Page 260-261
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What potential problem does STP (Spanning Tree Protocol) address?
Page 267
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In cut-through switching, which frame field does the switch never read?
Page 263
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You are asked to configure a backbone switch that connects servers supplying oceanic and atmospheric data to mariners and pilots around the world. Your network’s traffic load is very high at all times, day and night. What type of switching do you configure the switch to use?
Page 263
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Which of the following devices separates collision domains?
Pages 258-270
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Suppose your company’s network contains two separate VLANs. Computer A is on the
Customer Service VLAN and Computer B is on the Warehouse VLAN. Besides a Layer 2 switch, what device is required for Computer A and Computer B to exchange data?
Page 264
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Which of the following devices can act as a gateway?
Page 276

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Why can’t routers forward packets as quickly as bridges can?
Page 270
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In STP, what device acts as a guide to setting the best paths between switches?
Page 267
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What switching technique allows you to funnel traffic belonging to more than one VLAN through a single switch interface?
Page 265
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Which of the following identifies the VLAN to which each piece of data belongs?
Page 265 (bottom)
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Which of the following routing protocols is used on the Internet’s backbone?
Page 274 - bottom
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Which of the following types of routing protocols allows routers to exchange information about best paths with their neighboring routers only?
Page 274
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Why is a large, busy network more likely to use dynamic routing?
Page 272
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A packet on a network using the RIP routing method has been passed from one connectivity device to another 15 times. What happens when it gets passed to one more device?
Page 274
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What is the main difference between a Layer 3 switch and a router?
Page 270
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At which layers of the OSI model are gateways capable of functioning?
Page 276