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CCNA v3 Semester 1 Module 10 Routing Fundamentals and Subnets

By Tommy Riley,2014-06-17 08:18
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CCNA v3 Semester 1 Module 10 Routing Fundamentals and Subnets ...

CCNA v3 Semester 1 Module 10 Routing Fundamentals and Subnets

    CCNA v3 Semester 1 Module 10 Routing Fundamentals and Subnets ....................... 1 Overview ............................................................................................................... 1 10.1 Routed Protocol ......................................................................................... 2

    10.1.1 Routable and routed protocols ............................................................ 2

    10.1.2 IP as a routed protocol ........................................................................ 3

    10.1.3 Packet propagation and switching within a router ............................... 4

    10.1.4 Connectionless and connection-oriented delivery ............................... 5

    10.1.5 Anatomy of an IP packet .................................................................... 6 10.2 IP Routing Protocols .................................................................................. 8

    10.2.1 Routing overview ............................................................................... 8

    10.2.2 Routing versus switching.................................................................. 10

    10.2.3 Routed versus routing protocols ....................................................... 12

    10.2.4 Path determination ........................................................................... 13

    10.2.5 Routing tables .................................................................................. 15

    10.2.6 Routing algorithms and metrics ........................................................ 16

    10.2.7 IGP and EGP .................................................................................... 17

    10.2.8 Link state and distance vector ........................................................... 18

    10.2.9 Routing protocols ............................................................................. 19 10.3 The Mechanics of Subnetting ................................................................... 20

    10.3.1 Classes of network IP addresses ....................................................... 20

    10.3.2 Introduction to and reason for subnetting .......................................... 20

    10.3.3 Establishing the subnet mask address ............................................... 21

    10.3.4 Applying the subnet mask ................................................................ 23

    10.3.5 Subnetting Class A and B networks .................................................. 24

    10.3.6 Calculating the resident subnetwork through ANDing ...................... 25

    Summary ......................................................................................................... 26

Overview

    Internet Protocol (IP) is the main routed protocol of the Internet. IP addresses are used to route packets from a source to a destination through the best available path. The

    propagation of packets, encapsulation changes, and connection-oriented and

    connectionless protocols are also critical to ensure that data is properly transmitted to its destination. This module will provide an overview for each.

The difference between routing and routed protocols is a common source of confusion.

    The two words sound similar but are quite different. Routers use routing protocols to

    build tables that are used to determine the best path to a host on the Internet.

Not all organizations can fit into the three class system of A, B, and C addresses.

    Flexibility exists within the class system through subnets. Subnets allow network

    administrators to determine the size of the network they will work with. After they

    decide how to segment their networks, they can use subnet masks to determine the

    location of each device on a network.

This module covers some of the objectives for the CCNA 640-801, INTRO 640-821,

    and ICND 640-811 exams.

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    Students who complete this module should be able to perform the following tasks:

; Describe routed protocols

    ; List the steps of data encapsulation in an internetwork as data is routed to Layer

    3 devices

    ; Describe connectionless and connection-oriented delivery

    ; Name the IP packet fields

    ; Describe how data is routed

    ; Compare and contrast different types of routing protocols

    ; List and describe several metrics used by routing protocols

    ; List several uses for subnetting

    ; Determine the subnet mask for a given situation

    ; Use a subnet mask to determine the subnet ID

10.1 Routed Protocol

    10.1.1 Routable and routed protocols

This page will define routed and routable protocols.

    A protocol is a set of rules that determines how computers communicate with each other across networks. Computers exchange data messages to communicate with each other. To accept and act on these messages, computers must have sets of rules that determine how a message is interpreted. Examples include messages used to establish a connection to a remote machine, e-mail messages, and files transferred over a network.

A protocol describes the following:

; The required format of a message

    ; The way that computers must exchange messages for specific activities

    A routed protocol allows the router to forward data between nodes on different networks.

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    A routable protocol must provide the ability to assign a network number and a host number to each device. Some protocols, such as IPX, require only a network number. These protocols use the MAC address of the host for the host number. Other protocols, such as IP, require an address with a network portion and a host portion. These protocols also require a network mask to differentiate the two numbers. The network address is obtained by ANDing the address with the network mask.

    The reason that a network mask is used is to allow groups of sequential IP addresses to be treated as a single unit. If this grouping were not allowed, each host would have to be mapped individually for routing. This would be impossible, because according to the Internet Software Consortium there are approximately 233,101,500 hosts on the Internet.

10.1.2 IP as a routed protocol

This page describes the features and functions of IP.

    IP is the most widely used implementation of a hierarchical network-addressing scheme. IP is a connectionless, unreliable, best-effort delivery protocol. The term connectionless means that no dedicated circuit connection is established prior to transmission. IP determines the most efficient route for data based on the routing protocol. The terms unreliable and best-effort do not imply that the system is unreliable and does not work well. They indicate that IP does not verify that data sent on the network reaches its destination. If required, verification is handled by upper layer protocols.

    As information flows down the layers of the OSI model, the data is processed at each layer. At the network layer, the data is encapsulated into packets.

    These packets are also known as datagrams. IP determines the contents of the IP packet header, which includes address information. However, it is not concerned with the actual data. IP accepts whatever data is passed down to it from the upper layers.

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10.1.3 Packet propagation and switching within a router

    This page will explain the process that occurs as a packet moves through a network.

    As a packet travels through an internetwork to its final destination, the Layer 2 frame headers and trailers are removed and replaced at every Layer 3 device. This is because Layer 2 data units, or frames, are for local addressing. Layer 3 data units, or packets, are for end-to-end addressing.

    Layer 2 Ethernet frames are designed to operate within a broadcast domain with the MAC address that is burned into the physical device. Other Layer 2 frame types include PPP serial links and Frame Relay connections, which use different Layer 2 addressing schemes. Regardless of the type of Layer 2 addressing used, frames are designed to operate within a Layer 2 broadcast domain. When the data is sent to a Layer 3 device the Layer 2 information changes.

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    As a frame is received at a router interface, the destination MAC address is extracted. The address is checked to see if the frame is directly addressed to the router interface, or if it is a broadcast. In either situation, the frame is accepted. Otherwise, the frame is discarded since it is destined for another device on the collision domain.

    The CRC information is extracted from the frame trailer of an accepted frame. The CRC is calculated to verify that the frame data is without error.

    If the check fails, the frame is discarded. If the check is valid, the frame header and trailer are removed and the packet is passed up to Layer 3. The packet is then checked to see if it is actually destined for the router, or if it is to be routed to another device in the internetwork. If the destination IP address matches one of the router ports, the Layer 3 header is removed and the data is passed up to the Layer 4. If the packet is to be routed, the destination IP address will be compared to the routing table. If a match is found or there is a default route, the packet will be sent to the interface specified in the matched routing table statement. When the packet is switched to the outgoing interface, a new CRC value is added as a frame trailer, and the proper frame header is added to the packet. The frame is then transmitted to the next broadcast domain on its trip to the final destination.

10.1.4 Connectionless and connection-oriented delivery

    This page will introduce two types of delivery systems, which are connectionless and connection-oriented.

    These two services provide the actual end-to-end delivery of data in an internetwork.

    Most network services use a connectionless delivery system. Different packets may take different paths to get through the network. The packets are reassembled after they arrive at the destination. In a connectionless system, the destination is not contacted before a packet is sent. A good comparison for a connectionless system is a postal

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    system. The recipient is not contacted to see if they will accept the letter before it is sent. Also, the sender does not know if the letter arrived at the destination.

    In connection-oriented systems, a connection is established between the sender and the recipient before any data is transferred. An example of a connection-oriented network is the telephone system. The caller places the call, a connection is established, and then communication occurs.

    Connectionless network processes are often referred to as packet-switched processes. As the packets pass from source to destination, packets can switch to different paths, and possibly arrive out of order. Devices make the path determination for each packet based on a variety of criteria. Some of the criteria, such as available bandwidth, may differ from packet to packet.

    Connection-oriented network processes are often referred to as circuit-switched processes. A connection with the recipient is first established, and then data transfer begins. All packets travel sequentially across the same physical or virtual circuit.

    The Internet is a gigantic, connectionless network in which the majority of packet deliveries are handled by IP. TCP adds Layer 4, connection-oriented reliability services to IP.

10.1.5 Anatomy of an IP packet

    IP packets consist of the data from upper layers plus an IP header. This page will discuss the information contained in the IP header:

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    ; Version Specifies the format of the IP packet header. The 4-bit version field

    contains the number 4 if it is an IPv4 packet and 6 if it is an IPv6 packet.

    However, this field is not used to distinguish between IPv4 and IPv6 packets.

    The protocol type field present in the Layer 2 envelope is used for that. ; IP header length (HLEN) Indicates the datagram header length in 32-bit

    words. This is the total length of all header information and includes the two

    variable-length header fields.

    ; Type of service (ToS) 8 bits that specify the level of importance that has been

    assigned by a particular upper-layer protocol.

    ; Total length 16 bits that specify the length of the entire packet in bytes. This

    includes the data and header. To get the length of the data payload subtract the

    HLEN from the total length.

    ; Identification 16 bits that identify the current datagram. This is the sequence

    number.

    ; Flags A 3-bit field in which the two low-order bits control fragmentation. One

    bit specifies if the packet can be fragmented and the other indicates if the packet

    is the last fragment in a series of fragmented packets.

    ; Fragment offset 13 bits that are used to help piece together datagram

    fragments. This field allows the previous field to end on a 16-bit boundary. ; Time to Live (TTL) A field that specifies the number of hops a packet may

    travel. This number is decreased by one as the packet travels through a router.

    When the counter reaches zero the packet is discarded. This prevents packets

    from looping endlessly.

    ; Protocol 8 bits that indicate which upper-layer protocol such as TCP or UDP

    receives incoming packets after the IP processes have been completed. ; Header checksum 16 bits that help ensure IP header integrity.

    ; Source address 32 bits that specify the IP address of the node from which the

    packet was sent.

    ; Destination address 32 bits that specify the IP address of the node to which

    the data is sent.

    ; Options Allows IP to support various options such as security. The length of

    this field varies.

    ; Padding Extra zeros are added to this field to ensure that the IP header is

    always a multiple of 32 bits.

    ; Data Contains upper-layer information and has a variable length of up to 64

    bits.

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    While the IP source and destination addresses are important, the other header fields have made IP very flexible. The header fields list the source and destination address information of the packet and often indicate the length of the message data. The information for routing the message is also contained in IP headers, which can get long and complex

10.2 IP Routing Protocols

    10.2.1 Routing overview

    This page will discuss routing and the two main functions of a router.

    Routing is an OSI Layer 3 function. Routing is a hierarchical organizational scheme that allows individual addresses to be grouped together. These individual addresses are treated as a single unit until the destination address is needed for final delivery of the data. Routing finds the most efficient path from one device to another. The primary device that performs the routing process is the router.

The following are the two key functions of a router:

    ; Routers must maintain routing tables and make sure other routers know of

    changes in the network topology. They use routing protocols to communicate

    network information with other routers.

    ; When packets arrive at an interface, the router must use the routing table to

    determine where to send them. The router switches the packets to the

    appropriate interface, adds the frame information for the interface, and then

    transmits the frame.

    A router is a network layer device that uses one or more routing metrics to determine the optimal path along which network traffic should be forwarded. Routing metrics are values that are used to determine the advantage of one route over another. Routing protocols use various combinations of metrics to determine the best path for data.

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    Routers interconnect network segments or entire networks. Routers pass data frames between networks based on Layer 3 information. Routers make logical decisions about the best path for the delivery of data. Routers then direct packets to the appropriate output port to be encapsulated for transmission.

    Stages of the encapsulation and de-encapsulation process occur each time a packet transfers through a router. The router must de-encapsulate the Layer 2 data frame to access and examine the Layer 3 address. As shown above , the complete process of sending data from one device to another involves encapsulation and de-encapsulation on all seven OSI layers. The encapsulation process breaks up the data stream into segments, adds the appropriate headers and trailers, and then transmits the data. The de-encapsulation process removes the headers and trailers and then recombines the data into a seamless stream.

    This course focuses on the most common routable protocol, which is IP. Other examples of routable protocols include IPX/SPX and AppleTalk. These protocols provide Layer 3 support. Non-routable protocols do not provide Layer 3 support. The

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    most common non-routable protocol is NetBEUI. NetBEUI is a small, fast, and efficient protocol that is limited to frame delivery within one segment.

10.2.2 Routing versus switching

    This page will compare and contrast routing and switching. Routers and switches may seem to perform the same function. The primary difference is that switches operate at Layer 2 of the OSI model and routers operate at Layer 3. This distinction indicates that routers and switches use different information to send data from a source to a destination.

    The relationship between switching and routing can be compared to local and long-distance telephone calls. When a telephone call is made to a number within the same area code, a local switch handles the call. The local switch can only keep track of its local numbers. The local switch cannot handle all the telephone numbers in the world. When the switch receives a request for a call outside of its area code, it switches the call to a higher-level switch that recognizes area codes. The higher-level switch then switches the call so that it eventually gets to the local switch for the area code dialed.

    The router performs a function similar to that of the higher-level switch in the telephone example. Figure shows the ARP tables for Layer 2 MAC addresses and

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