Understanding IP Routing

The primary objective of this book is to provide elaborate guidance for troubleshooting Internet Protocol (IP) routing problems on Cisco routers. In this regard, the subsequent text covers well-known routing protocols such as the following:

• Open Shortest Path First Protocol (OSPF)

• Integrated Intermediate System-to-Intermediate System Protocol (IS-IS)

• Border Gateway Protocol (BGP)

• Protocol Independent Multicast (PIM) for multicast routing

This chapter presents an introduction to IP routing and provides insights to related con-cepts, such as IP addressing and various classifications of IP routing protocols. The chapter also provides a high-level overview of implementation and configuration concepts, such as route filtering and redistribution.

The Transmission Control Protocol/Internet Protocol (TCP/IP) suite of protocols is the underlying technology for information exchange on the Internet. TCP/IP uses a layering approach for computer communications similar to the Open System Interconnection (OSI) reference model, but with fewer than seven layers. Figure 1-1 shows the OSI reference model and the TCP/IP stack side by side. Related layers between the two stacks are indicated in the figure.

Figure 1-1. OSI Reference Model and TCP/IP Stack

Figure 1-1. OSI Reference Model and TCP/IP Stack

IP operates at the Internet layer of the TCP/IP suite, which corresponds to the network layer of the OSI reference model. IP provides connectionless data-delivery services, which involve transmission of information from one part of a network to another in units of data known as packets or datagrams. The essence of the datagram delivery service model is that a permanent pre-established end-to-end path is not required for data transfer between two points in a network. In a packet-based network, each router in the transmission path makes independent local decisions regarding the optimal forwarding path toward the destination for any transit packet. The decision-making is based on forwarding intelligence gathered either dynamically by means of a routing protocol or manually programmed static routes.

Addressing is an important aspect of the data-forwarding process. For any directed com-munication, there is a source and a destination. Addressing allows the target destination to be specified by the source and allows the destination node to also identify the source. Addressing is even more important in the datagram delivery mode of operation because, as in IP forwarding, the data path for any transmission is not nailed through the intermediate nodes between the source and destination.

As mentioned previously, within the IP datagram services infrastructure, information that is to kn frnnrmiffn/H frnrY\ r\r*n r\ i\ 4-/-v -»nrvf knr -Pi »-r">l- it- krrvl/nn /-vi a / r> infn m/'l/Afr C's/'k r>nrl/nf k-sr


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