Curs PC si internet cap 11.5 Routing Protocols

11.5 Routing Protocols 11.5.1 Examples of routing protocols Instructor Note The concept of routing protocols is introduced. Students should be encouraged not to confuse routing protocols with routed protocols -- routed protocols allow packets to be routed; routing protocols are languages routers speak to each other to constantly keep each other informed about the topology of the network. Routing protocols (Note: Do not confuse with routed protocols.) determine the paths that routed protocols follow to their destinations. Examples of routing protocols include the Routing Information Protocol (RIP), the Interior Gateway Routing Protocol (IGRP), the Enhanced Interior Gateway Routing Protocol (EIGRP), and Open Shortest Path First (OSPF). Routing protocols enable routers that are connected, to create a map, internally, of other routers in the network or on the Internet. This allows routing (i.e. selecting the best path, and switching) to occur. Such maps become part of each router's routing table. 11.5 Routing Protocols 11.5.2 Definition of routing protocol Instructor Note Routing protocol is defined. To make the example concrete, the long-established and widely available routing protocol RIP is studied as a simple example of a routing protocol. RIP represents some of the major aspects of routing protocols: use of a metric to make routing decisions (in RIPs case, hop count) and an update process (to ensure timely communication amongst the routers). Emphasize to students that without routing protocols to update each other about the state of the network topology, disruptions in that topology (which grow more likely as the internetwork grows larger) become fatal to packets (they become undeliverable for lack of a path) trying to traverse the network. Routing protocols, when properly running, assure the routers have a consistent and up-to-date way to decide how to choose the best path. Routers use routing protocols to exchange routing tables and to share routing information. Within a network, the most common protocol used to transfer routing information between routers, located on the same network, is Routing Information Protocol (RIP). This Interior Gateway Protocol (IGP) calculates distances to a destination host in terms of how many hops (i.e. how many routers) a packet must pass through. RIP enables routers to update their routing tables at programmable intervals, usually every 30 seconds. One disadvantage of routers that use RIP is that they are constantly connecting to neighboring routers to update their routing tables, thus creating large amounts of network traffic. RIP allows routers to determine which path to use to send data. It does so by using a concept known as distance-vector. Whenever data goes through a router, and thus, through a new network number, this is considered to be equal to one hop. A path which has a hop count of four indicates that data traveling along that path would have to pass through four routers before reaching the final destination on the network. If there are multiple paths to a destination, the path with the least number of hops would be the path chosen by the router.Because hop count is the only routing metric used by RIP, it doesnt necessarily select the fastest path to a destination. A metric is a measurement for making decisions and soon you will learn that other routing protocols use many other metrics besides hop count to find the best path for data to travel. Nevertheless, RIP remains very popular, and is still widely implemented. This may be due primarily to the fact that it was one of the earliest routing protocols to be developed. One other problem posed by the use of RIP is that sometimes a destination may be located too far away to be reachable. When using RIP, the maximum number of hops that data can be forwarded through is fifteen. The destination network is considered unreachable if it is more than fifteen router hops away. 11.5 Routing Protocols 11.5.3 Routing encapsulation sequence Instructor Note It is important for students to realize that the router de-encapsulates packets up to the network layer to examine the destination network layer address. Students should be familiar with de-encapsulation for chapter 2, since all receiving (destination) hosts have to go through this process. If it finds the address in its routing table, it choose the best path to get to that destination, switches the packet to the proper interface, re-encapsulates the packet and sends it on its way. However, if there is no match in the routing table, the packet is dropped. At the data link layer, an IP datagram is encapsulated into a frame. The datagram, including the IP header, is treated as data. A router receives the frame, strips off the frame header, then checks the destination IP address in the IP header. The router then looks for that destination IP address in its routing table, encapsulates the data in a data link layer frame, and sends it out to the appropriate interface. If it does not find the destination IP address, it may drop the packet. 11.5 Routing Protocols 11.5.4 Multi-protocol routing Instructor Note The students are introduced to the definition of multi-protocol routing. This flexible feature of routers allows them to inter-connect a diverse array of networks. The reality of the computing world is diversity -- many different vendors and protocols -- so it is important for routers to be able to process. The analogy is that the router is "multilingual." Routers are capable of supporting multiple independent routing protocols, and of maintaining routing tables for several routed protocols, concurrently. This capability allows a router to deliver packets from several routed protocols over the same data links.