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Understanding IP Routing

Troubleshooting IP Routing Protocols offers you a full understanding of invaluable troubleshooting techniques that help keep your network operating at peak performance. Whether you are looking to hone your support skills or to prepare for the challenging CCIE exams, this essential reference shows you how to isolate and resolve common network failures and to sustain optimal network operation.

Excerpt:

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 OSI reference model, but with fewer than seven layers.

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 communication, 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 be transmitted from one device to another first is broken down into packets. Each packet has an IP header, a transport layer (TCP or UDP) header, and a payload, which is a piece of the original information. Each IP packet is self-contained and independently is forwarded to the destination through the chain of intermediate devices that might be along the path of transmission.

The routers in the network depend on a routing protocol or static configuration to forward the datagrams in a stream to their intended destination. For any destination address, each node in the data path worries about only the outgoing interface or link along a locally determined optimal path to the destination (or as specified by a special forwarding policy).

The IP forwarding process frequently is described as a hop-by-hop destination-based forwarding mechanism. This means the routers at each hop along the data path normally forward packets based on the destination address. However, modern routers also can use policy-based criteria, such as the source address in a packet to direct the forwarding.

At the destination, packets belonging to the same stream are reassembled into the original information. This process of forwarding a packet from one node to the other in a connectionless network on the Layer 3 address (IP address, in this case) also is referred to as routing. Routers are specialized network devices with acquired intelligence.

So how do routers decide where and how to forward packets traversing the internetwork? Well, this is done in a couple of ways. As alluded to previously, routers can be manually preprogrammed with predetermined path information known as static routes, or they can run applications that facilitate the learning and sharing of routing information automatically. Obtaining routing information by the latter method is referred to as dynamic routing.

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