This table contains the learned networks and associated ports for those networks. Routers use routing protocols to manage information received from other routers, information learned from the configuration of its own interfaces, along with manually configured routes.The routing protocol learns all available routes, places the best routes into the routing table, and removes routes when they are no longer valid. The router uses the information in the routing table to forward routed protocol packets. The routing algorithm is fundamental to dynamic routing. Whenever the topology of a network changes because of growth, reconfiguration, or failure, the network knowledgebase must also change. The network knowledgebase needs to reflect an accurate consistent view of the new topology. When all routers in an internetwork are operating with the same knowledge, the internetwork is said to have converged. Fast convergence is desirable because it reduces the period of time in which routers would continue to make incorrect routing decisions. Autonomous systems (AS) provide the division of the global internetwork into smaller and more manageable networks. Each AS has its own set of rules and policies and an AS number that will uniquely distinguish it from other autonomous systems throughout the world. Web Links Introduction to Routing Protocols http://www.cisco.com/networkers/ nw00/pres/ 2204.pdf
Content 6.2 Dynamic Routing Overview 6.2.4 Identifying the classes of routing protocols Most routing algorithms can be classified into one of two categories: The distance vector routing approach determines the direction (vector) and distance to any link in the internetwork. The link-state approach, also called shortest path first, recreates the exact topology of the entire internetwork. Web Links Routing Algorithms http://www.broadband-help.com/ guestarticles/ routingalg/ Algorithm.htm
Content 6.2 Dynamic Routing Overview 6.2.5 Distance vector routing protocol features Distance vector routing algorithms pass periodic copies of a routing table from router to router. These regular updates between routers communicate topology changes. Distance vector based routing algorithms are also known as Bellman-Ford algorithms.Each router receives a routing table from its directly connected neighbor routers. Router B receives information from Router A. Router B adds a distance vector number (such as a number of hops), which increases the distance vector. Then Router B passes this new routing table to its other neighbor, Router C. This same step-by-step process occurs in all directions between neighbor routers. The algorithm eventually accumulates network distances so that it can maintain a database of network topology information. However, distance vector algorithms do not allow a router to know the exact topology of an internetwork as each router only sees its neighbor routers. Each router that uses distance vector routing begins by identifying its own neighbors. The interface that leads to each directly connected network is shown as having a distance of 0. As the distance vector network discovery process proceeds, routers discover the best path to destination networks based on the information they receive from each neighbor. Router A learns about other networks based on the information that it receives from Router B. Each of the other network entries in the routing table has an accumulated distance vector to show how far away that network is in a given direction. Routing table updates occur when the topology changes. As with the network discovery process, topology change updates proceed step-by-step from router to router. Distance vector algorithms call for each router to send its entire routing table to each of its adjacent neighbors. The routing tables include information about the total path cost as defined by its metric and the logical address of the first router on the path to each network contained in the table. An analogy of distance vector could be the signs found at a highway intersection. A sign points towards a destination and indicates the distance to the destination. Further down the highway, another sign points toward the destination, but now the distance is shorter. As long as the distance is shorter, the traffic is following the best path. Web Links Routing Protocols - Distance Vector http://www.firewall.cx/ index.php?c=distance_vector
Content 6.2 Dynamic Routing Overview 6.2.6 Link-state routing protocol features The second basic algorithm used for routing is the link-state algorithm. Link-state algorithms are also known as Dijkstras algorithm or as SPF (shortest path first) algorithms. Link-state routing algorithms maintain a complex database of topology information. The distance vector algorithm has nonspecific information about distant networks and no knowledge of distant routers. A link-state routing algorithm maintains full knowledge of distant routers and how they interconnect.Link-state routing uses: Network discovery processes for link state routing
LSAs are exchanged between routers starting with directly connected networks for which they have direct information. Each router in parallel with the others constructs a topological database consisting of all the exchanged LSAs. The SPF algorithm computes network reachability. The router constructs this logical topology as a tree, with itself as the root, consisting of all possible paths to each network in the link-state protocol internetwork. It then sorts these paths Shortest Path First (SPF). The router lists the best paths and the interfaces to these destination networks in the routing table. It also maintains other databases of topology elements and status details. The router that first becomes aware of a link-state topology change forwards the information so that all other routers can use it for updates. This involves sending common routing information to all routers in the internetwork. To achieve convergence, each router keeps track of its neighbor routers, the router name, interface status, and the cost of the link to the neighbor. The router constructs an LSA packet that lists this information along with new neighbors, changes in link costs, and links that are no longer valid. The LSA packet is then sent out so that all other routers receive it. When the router receives an LSA, the database is updated with the most recent information and computes a map of the internetwork using the accumulated data and calculates the shortest path to other networks using the SPF algorithm. Each time an LSA packet causes a change to the link-state database, SPF recalculates the best paths and updates the routing table. Link-state concerns: Routers running link-state protocols require more memory and perform more processing than distance vector routing protocols. Routers must have sufficient memory to be able to hold all the information from the various databases, the topology tree, and the routing table. Initial link-state packet flooding consumes bandwidth. During the initial discovery process, all routers using link-state routing protocols send LSA packets to all other routers. This action floods the internetwork and temporarily reduces bandwidth available for routed traffic carrying user data. After this initial flooding, link-state routing protocols generally require only minimal bandwidth to