Cisco Router Internetworking

by Paul T Ammann CCNA

Overview

This text presents readers with the concepts and commands required to configure Cisco routers in multiprotocol internetworks. Examples, excercises and tests are included to allow readers to learn to qualify for Cisco certification.

Product Details

ISBN-13:	9780071356275
Publisher:	McGraw-Hill Companies, The
Publication date:	12/01/1999
Series:	McGraw-Hill Technical Expert
Pages:	804
Product dimensions:	7.50(w) x 9.25(h) x 1.60(d)

Read an Excerpt

Chapter 3: The Network Infrastructure

The network infrastructure is an important component in all network design, simply because, at the end of the day, it is those wires that carry the information. A well-thought-out network infrastructure not only provides reliable and fast delivery of that information, but is also able to adapt to changes, and grow as business expands.

Building a network infrastructure is a complex task, requiring information gathering, planning, designing, and modeling. Though it deals mainly with bits and bytes, it is more of an art than a science, because there are no hard and fast rules for building one. When you build a network infrastructure, you look more at the lower three layers of the OSI model, although many other factors need to be considered. There are many technologies available that you can use to build a network, and the challenge a network manager faces is to choose the correct one and the tool that comes with it. It is important to know the implications of selecting a particular technology, because the network manager ultimately decides what equipment is required. When selecting a piece of networking equipment, it is important to know at which layer of the OSI model the device functions. The functionality of the equipment is important because the equipment must conform to certain standards, it has to live up to the expectation of the application, and it has to perform tasks required by the blueprint-the network architecture.

The implementation of IP over different protocols depends on the mechanism used for mapping the IP addresses to the hardware addresses, or MAC address, at the data-link layer of the OSI model. Important aspects toconsider when using IP over any data-link protocol are:

• Address mapping-Different data-link layer protocols have different ways of mapping the IP address to the hardware address. In the TCP/IP protocol suite, the Address Resolution Protocol (ARP) is used for this purpose, and it works only in a broadcast network.

• Encapsulation and overheads-The encapsulation of the IP packets into the data link layer packet and the overheads incurred should be evaluated. Because different data-link layer protocols transport information differently, one may be more suitable than another.

• Routing-Routing is the process of transporting the IP packets from network to network, and is an important component in an IP network. Many protocols are available to provide intelligence in the routing of the IP protocol, some with sophisticated capabilities. The introduction of switching and some other data-link layer protocols has introduced the possibility of building switched paths in the network that can bypass the routing process. This saves network resources and reduces network delay by eliminating the slower process of routing that relies on software rather than on hardware or microcode switching mechanisms.

• Maximum Transmission Unit (MTU)-Another parameter that should be considered in the IP implementation over different data-link layer protocols is the maximum transmission unit (MTU) size. This refers to the size of the data frame (in bytes) that has to be transmitted to the destination through the network. A bigger MTU size means one can send more information within a frame, thus requiring a lower total number of packets to transmit a piece of information.

Different data-link layers have different MTU sizes for the operation of the network. If you connect two networks with different MTU sizes, then a process called fragmentation takes place and must be performed by an external device, such as a router. Fragmentation takes a larger packet and breaks it up into smaller ones so that it can be sent onto the network with a smaller MTU size. Fragmentation slows down the traffic flow and should be avoided as much as possible.

Technology

Besides having wires to connect all the devices together, you need to decide the way these devices connect, the protocol in which the devices should talk to each other. Various technologies are available, with differing standards and implementation.

In this section, a few popular technologies are covered and their characteristics are highlighted. These technologies cover LAN, WAN, as well as remote access area.

The Basics

It is important to understand the fundamentals of how data are transmitted in an IP network, so that the difference in how the various technologies work can be better understood. Each workstation connects to the network through a network interface card (NIC) that has a unique hardware address. At the physical layer, these workstations communicate with each other through the hardware addresses. IP, being a higher-level protocol in the OSI model, communicates through a logical address, which in this case is the IP address. When one workstation with an IP address of 10.1.1.1 wishes to communicate with another with the address 10.1.1.2, the NIC does not understand these logical addresses. Some mechanism must be implemented to translate the destination address 10.1.1.2 to a hardware address that the NIC can understand.

Broadcast versus Non-Broadcast Network

Generally, all networks can be grouped into two categories: broadcast and non-broadcast. The mechanism for mapping the logical address to the hardware address is different for these two groups of networks. The best way of describing a broadcast network is to imagine a teacher teaching a class. The teacher talks and every student listens. An example of a non-broadcast network would be a mail correspondence-at any time, only the sender and receiver of the mail know what the conversation is about, the rest of the people don't. Examples of broadcast networks are Ethernet, token ring, and FDDI, while examples of non-broadcast networks are frame relay and ATM.

It is important to differentiate the behavior of both broadcast and nonbroadcast networks, so that the usage and limitation can both be taken into consideration in the design of an IP network.

Address Resolution Protocol (ARP)

In a broadcast network, the ARP is used to translate the IP address to the hardware address of the destination host. Every workstation that runs the TCP/IP protocol keeps a table, called an ARP cache, containing the mapping of the IP address to the hardware address of the hosts with which it is communicating. When a destination entry is not found in the ARP cache, a broadcast, called ARP broadcast, is sent out to the network. All workstations located within the same network will receive this request and check the IP address entry in the request. If one of the workstations recognizes its own IP address in this request, it will proceed to respond with an ARP reply, indicating its hardware address. The originating workstation then stores this information and commences to send data through the newly learned hardware address.

ARP provides a simple and effective mechanism for mapping an IP address to a hardware address. However, in a large network, especially in a bridged environment, a phenomenon known as a broadcast storm can occur if workstations misbehave, assuming hundreds of workstations are connected to a LAN, and ARP is used to resolve the address mapping issue. If the workstation's ARP cache is too small, it means the workstation has to send more broadcasts to find out the hardware address of the destination. Having hundreds of workstations continuously sending out ARP broadcasts would soon render the LAN useless because nobody could send any data.

Proxy ARP

The standard ARP protocol does not allow the mapping of hardware addresses between two physically separated networks that are interconnected by a...

Table of Contents

Chapter 1: Internetworking Overview.
Chapter 2: The OSI Model.
Chapter 3: The Network Infrastructure.
Chapter 4: Cisco Router Configuration Basics.
Chapter 5: TCP/IP Addressing.
Chapter 6: TCP/IP routing Protocols.
Chapter 7: Implementing IP-Enhanced IGRP.
Chapter 8: Implementing BGP and Route Redistribution.
Chapter 9: Managing Traffic.
Chapter 10: IPX Overview and Addressing.
Chapter 11: Implementing NLSP and IPXWAN.
Chapter 12: Managing IPX Traffic.
Chapter 13: AppleTalk Overview and Addressing.
Chapter 14: Implementing Routing for AppleTalk.
Chapter 15: Implementing AppleTalk Enhanced IGRP.
Chapter 16: Managing AppleTalk Traffic.
Chapter 17: WAN Connections.
Chapter 18: WAN Scalability.
Chapter 19: x.25 Configuration.
Chapter 20: Frame Relay Configuration.
Chapter 21: PPP Configuration.
Chapter 22: ISDN Configuration.
Chapter 23: SMDS and ATM Configuration.
Chapter 24: Bandwidth on Demand.
Chapter 25: Transparent and Source-Router Bridging.
(and more...)