The Eigrp Packet Header
Figure 8.28 shows the EIGRP header, which begins every EIGRP packet. Figure 8.28. The EIGRP packet header. Figure 8.28. The EIGRP packet header. Version specifies the particular version of the originating EIGRP process. Although two software releases of EIGRP are currently available, 11 the version of the EIGRP process itself has not changed since its release. 11 Because of the improvements to its stability beginning with IOS 10.3(11), 11.0(8), and 11.1(3) use of the later version of EIGRP is...
Appendix D Answers to Review Questions
Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 Chapter 12 Chapter 14 1 What is the primary purpose of a LAN A The primary purpose of a local-area network is to allow resource sharing. The resources may be devices, applications, or information. Examples of shared resources are files, databases, e-mail, modems, and printers. A A protocol is an agreed-upon set of rules. In data communications, the rules usually govern a procedure or...
Case Study Policy Routing and Quality of Service Routing
Although Quality of Service (QoS) routing is outside the scope of this volume, it must be noted here that policy routing can be an integral part of QoS. Policy routing in conjunction with QoS is done by setting the Precedence or the Type of Service (TOS) bits of the TOS field in the IP headers of packets as they enter a router's interface.Figure 14.9 shows the bits of the TOS field. Although the TOS bits are seldom used in modern internetworks, the Precedence bits have found new life in QoS...
Case Study Route Tagging
Figure 14.17 shows a situation in which routes from several routing domains, each running a separate routing protocol, are being redistributed into a single transit domain running OSPF. On the other side of the OSPF cloud, the routes must be redistributed back into their respective domains. Route filters can be used at the egress points from the OSPF cloud into each domain to permit only the routes that belong to that domain. However, if each domain has many routes or if the routes within the...
The IP Packet Header
Figure 2.2 shows the format of the IP packet header, specified in RFC 791. Most fields in this packet have some importance to routing. Version identifies the I P version to which the packet belongs. This four-bit field is usually set to binary 0100 version 4 (IPv4) is in current, common use. A newer version of the protocol, not yet in widespread deployment, is version 6 (IPv6), sometimes referred to as next-generation IP(IPng). All currently assigned version numbers can be seen in Table 2.1,...
The Route Table
To understand the kind of information that exists in the route table, it is useful to begin with an examination of what happens when a framed packet arrives at one of a router's interfaces. The data-link identifier in the frame's destination address field is examined. If it contains either the identifier of the router's interface or a broadcast identifier, the router strips off the frame and passes the enclosed packet to the network layer. At the network layer, the destination address of the...
Configuring Route Filters
Using Distances to Set Router Preferences Chapter 11, Route Redistribution, presents several situations in which redistribution causes unwanted or inaccurate routes to exist in a particular router. For instance, in Figure 11.3 and the associated discussion, one or more routers choose a sub-optimal route through an internetwork. The problem in that example is that the routers prefer the lower administrative distance of IGRP to the administrative distance of RIP. More generally, any time routes...
Troubleshooting Static Routes
Followers of the assorted American political scandals of the past 30 or so years will have heard a congressional investigator ask the question, What did he know and when did he know it The same question serves an internetworking investigator well. When troubleshooting routing problems, the first step should almost always be to examine the route table. What does the router know Does the router know how to reach the destination in question Is the information in the route table accurate Knowing...
Appendix E Solutions to Configuration Problems
Chapter 2 Chapter 3 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 Chapter 13 Chapter 14 1 The first octet rule says that the highest class C address is 223, but it is known that for eight bits the highest decimal number is 255. There are two more classes Class D addresses are for multicast, and class E addresses are for experimental usage. Class D addresses have, as their first four bits, 1110. What is the decimal range of the first octet of class D addresses A If the...
Variable Length Subnet Masking
If a subnet mask can be individually associated with each destination address advertised throughout an internetwork, there is no reason why all the masks must be of equal length. That fact is the basis for VLSM. A simple application of VLSM is shown in Figure 7.4. Each data link of the internetwork shown must have a uniquely identifiable subnet address, and each subnet address must contain enough host addresses to accommodate the devices attached to the data link. Figure 7.4. Using VLSM, the...
Building an Adjacency
Neighbors on point-to-point, point-to-multipoint, and virtual link networks always become adjacent unless the parameters of their Hellos don't match. On broadcast and NBMA networks, the DR and BDR become adjacent with all neighbors, but no adjacencies will exist between DRothers. The adjacency building process uses three OSPF packet types 1. Database Description packets (type 2) 2. Link State Request packets (type 3) 3. Link State Update packets (type 4) Table 9.2. Input events for Figures...
Configuring Static Routes
The route table acquires information in two ways. The information may be entered manually, by means of a static route entry, or automatically by one of several systems of automatic information discovery and sharing known as dynamic routing protocols. The bulk of this book concerns dynamic IP routing protocols, but this discussion of static route configuration will prepare you to understand the subsequent chapters. More to the point, static routing is preferred over dynamic routing in certain...
The Interface State Machine
An OSPF-enabled interface will transition through several states before it becomes fully functional. Those states are Down, Point-to-Point, Waiting, DR, Backup, DRother, and Loopback. This is the initial interface state. The interface is not functional, all interface parameters are set to their initial values, and no protocol traffic is transmitted or received on the interface. This state is applicable only to interfaces connected to point-to-point, point-to-multipoint, and virtual link network...
The Hostto Host Layer
The host-to-host layer of the TCP IP protocol is aptly named. Whereas the internet layer is responsible for the logical paths between networks, the host-to-host layer is responsible for the full logical path between two hosts on disparate networks 121 . From another viewpoint, the host-to-host layer is an interface to the lower layers of the protocol suite, freeing applications from any concern about how their data is actually being delivered. 121 Similarly, it can be said that the equivalent...
Case Study Route Maps and Redistribution
A route map can be used with redistribution by adding a call to the route map in the redistribute command. Figure 14.12 shows an internetwork in which IS-IS and OSPF routes are being mutually redistributed at router Zippy. Of the network and subnet addresses listed in the illustration, only the ones whose third octet is odd-numbered are to be redistributed. Zippy's configuration is Figure 14.12. The OSPF and IS-IS routes are being mutually redistributed. Route maps can be used with the...
