Partial Mesh Topology

A WAN could also be developed with a partial mesh topology. Partial meshes are highly flexible topologies that can take a variety of very different configurations. The best way to describe a partial mesh topology is that the routers are much more tightly coupled than in any of the basic topologies but are not fully interconnected. Fully interconnected routers form a fully meshed topology. Figure 13-4 shows the partial mesh topology. Figure 13-4 A partial mesh topology. Figure 13-4 A partial...

Pv4 Address Formats

IP was standardized in September 1981. Its address architecture was as forward looking as could be expected given the state of computing at that time. The basic IP address was a 32-bit binary number that was compartmentalized into four 8-bit binary numbers, or octets. To facilitate human usage, IP's machine-friendly binary addresses were converted into a more familiar number system base 10. Each of the four octets in the IP address is represented by a decimal number, from 0 to 255, and...

OSPF Data Structures

OSPF is a fairly complex routing protocol with many performance- and stability-enhancing features. Therefore, it shouldn't be a surprise to find that it uses an extensive array of data structures. Each structure, or message type, is intended to perform a specific task. All of them share a common header, known as the OSPF header. The OSPF header is 24 octets long and has the following fields Version Number The first octet of an OSPF header is allocated to the identification of the version...

The Mechanics of Logical Adjacency

To enable the logical adjacency of layers, each layer of the originating machine's protocol stack adds a header to the data received from the layer above it. This header can be recognized and used by only that layer or its counterparts on other machines. The receiving machine's protocol stack removes the headers, one layer at a time, as the data is passed up to its application. Figure 1-7 illustrates this process. Note that Figure 1-7 presents a theoretical view of layered communications rather...

Limitations of RIP2

Despite its overhaul, RIP-2 couldn't compensate for all its predecessors' limitations. In fairness to the creators of RIP-2, they didn't seek to make RIP-2 anything but a modernized RIP. This included maintaining its original purpose as an IGP for use in small networks or autonomous systems. Therefore, all the original functional limitations designed into RIP also apply to RIP-2. The critical differences are that RIP-2 can be used in networks that require either support for authentication and...

Equal Cost Load Balancing

Equal-cost load balancing, as its name implies, is the balancing of a traffic load across redundant links of equal cost. Figure 10-3 illustrates this in a small network, and the contents of Router A's routing table are presented in Table 10-4. In this table, you can see that there are multiple ports that Router A can use to get to the gateway router. These ports are called S0 and S1, or serial ports 0 and 1. Figure 10-3 Load balancing across equal links. Figure 10-3 Load balancing across equal...

The Internet Layer

The Internet layer of IPv4 consists of all the protocols and procedures necessary to allow data communications between hosts to traverse multiple networks. This means that the data-bearing packets must be routable. IP makes data packets routable. Figure 5-4 illustrates the structure of the IP header, as well as the sizes of its fields. Figure 5-4 The structure of an IP header. The IP header has the following size and fields IP Version Number The first 4 bits of the IP header identify the...

Computer Based Routers

Traditional, standalone routers are hardware specific You purchase a specialized physical platform, including a chassis, sheet metal, power supply, CPU, memory, I O ports, and a motherboard together with the routing engine. These components are described in Routers and WANs. In a standalone router, the routing engine is an integral part of the unit. It is not separable, nor portable, from the standalone router. In a computer-based router, the routing engine is executable software designed to...

Backward Compatibility with IGRP

Although EIGRP was developed as a more up-to-date and efficient alternative to IGRP, it was also explicitly an extension of IGRP. Consequently, the two are designed to be completely compatible. These two routing protocols even share the same distance-vector routing technology EIGRP uses the same composite routing metric as IGRP. EIGRP also supports all the same distance vectors, and their mathematical weights, as does IGRP. EIGRP also uses IGRP's Variance feature to provide unequal-cost load...

Advertising Network Addresses

Each of the hosts in the Internet needed to be uniquely identifiable. In the Internet's two-level hierarchy, this required an address with two parts Together, these two types of addresses could uniquely identify any and all machines connected via the Internet. It is possible that the needs of a small, networked community could be satisfied with just host addresses, as is the case with LANs. Network addresses, however, are necessary for end systems on different networks to communicate with each...

The Seven Layers

The OSI model categorizes the various processes needed in a communications session into seven distinct functional layers. The layers are organized based on the natural sequence of events that occur during a communications session. Figure 1-1 illustrates the OSI reference model. Layers 1-3 provide network access, and Layers 4-7 are dedicated to the logistics of supporting end-to-end communications. Figure 1-1 The OSI reference model. Figure 1-1 The OSI reference model. The bottom layer, or Layer...

IP Routing Fundamentals

An Introduction to Internetworking Understanding Internetwork Addresses Internet Protocols Versions The Mechanics of Routing Protocols Internetworking with Dissimilar Protocols HOME CONTENTS PREVI& US NEXT GLOSSARY FEEDBACK SEARCH HELP Copyright 1989-1999 Cisco Systems Inc. Welcome to the employee only Cisco Press web site. The above Welcome page link presents a FAQ sheet for Cisco Press, including information about how you can buy Cisco Press books . New information on the Cisco Press...

The Media Access Domain

A media access domain consists of all the devices connected to a LAN that must share the LAN's bandwidth. The name and nature of this domain depends on the media access methodology employed in a LAN. The two primary methodologies for regulating media access are contention and token passing. Other media access arbitration techniques exist, but these two account for the vast majority of existing LANs. More importantly, they will adequately demonstrate the differences between a media access domain...

Pv4 Addressing Scheme

IPv4 uses a 32-bit binary addressing scheme to identify networks, network devices, and network-connected machines. These addresses, known as IP addresses, are strictly regulated by the Internet Assigned Numbers Authority (IANA) to ensure their uniqueness in the Internet. This function is currently being transitioned out of the hands of the U.S. government in favor of a private organization. This reflects both the desire of the U.S. government to control its expenses, and the undeniably global...