sometimes chooses slow routes because it ignores critical factors such as bandwidth in route determination. OSPF overcomes these limitations and proves to be a robust and scalable routing protocol suitable for the networks of today. OSPF can be used and configured as a single area for small networks. It can also be used for large networks. OSPF routing scales to large networks if hierarchical network design principles are used. Large OSPF networks use a hierarchical design. Multiple areas connect to a distribution area, area 0, also called the backbone. This design approach allows for extensive control of routing updates. Defining areas reduces routing overhead, speeds up convergence, confines network instability to an area and improves performance. Web Links Open Shortest Path First (OSPF) http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ito_doc/ospf.htm
Content 2.2 Single Area OSPF Concepts 2.2.2 OSPF terminology As a link-state protocol, OSPF operates differently from distance vector routing protocols. Link-state routers identify neighboring routers and then communicate with the identified neighbors. OSPF has its own terminology. The new terms are shown in Figure . Information is gathered from OSPF neighbors about the status, or links, of each OSPF router. This information is flooded to all its neighbors. Flooding is a process that sends information out all ports, with the exception of the port on which the information was received. An OSPF router advertises its own link states and passes on received link states. The routers process the information about link-states and build a link-state database. Every router in the OSPF area will have the same link-state database. Every router has the same information about the state of the links and the neighbors of every other router. Then each router runs the SPF algorithm on its own copy of the database. This calculation determines the best route to a destination. The SPF algorithm adds up the cost, which is a value that is usually based on bandwidth. The lowest cost path is added to the routing table, which is also known as the forwarding database. OSPF routers record information about their neighbors in the adjacency database. To reduce the number of exchanges of routing information among several neighbors on the same network, OSPF routers elect a Designated Router (DR) and a Backup Designated Router (BDR) that serve as focal points for routing information exchange. Interactive Media Activity Crossword Puzzle: OSPF Terminology When the student has completed this activity, the student will understand the different OSPF terminology. Web Links Open Shortest Path First (OSPF) http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ito_doc/ospf.htm
Content 2.2 Single Area OSPF Concepts 2.2.3 Comparing OSPF with distance vector routing protocols OSPF uses link-state technology, compared with distance vector technology such as RIP. Link-state routers maintain a common picture of the network and exchange link information upon initial discovery or network changes. Link-state routers do not broadcast their routing tables periodically as distance vector protocols do. Therefore, link-state routers use less bandwidth for routing table maintenance. RIP is appropriate for small networks, and the best path is based on the lowest number of hops. OSPF is appropriate for the needs of large scalable internetworks, and the best path is determined by speed. RIP and other distance vector protocols use simple algorithms to compute best paths. The SPF algorithm is complex. Routers implementing distance vector routing may need less memory and less powerful processors than those running OSPF. OSPF selects routes based on cost, which is related to speed. The higher the speed, the lower the OSPF cost of the link. OSPF selects the fastest loop-free path from the shortest-path first tree as the best path in the network. OSPF guarantees loop-free routing. Distance vector protocols may cause routing loops. If links are unstable, flooding of link-state information can lead to unsynchronized link-state advertisements and inconsistent decisions among routers. OSPF addresses the following issues: In large networks RIP convergence can take several minutes since the routing table of each router is copied and shared with directly connected routers. After initial OSPF convergence, maintaining a converged state is faster because only the changes in the network are flooded to other routers in an area. OSPF supports VLSMs and therefore is referred to as a classless protocol. RIP v1 does not support VLSMs, however, RIP v2 does support VLSMs. RIP considers a network that is more than 15 routers away to be unreachable because the number of hops is limited to 15. This limits RIP to small topologies. OSPF has no size limits and is suitable for intermediate to large networks. RIP selects a path to a network by adding one to the hop count reported by a neighbor. It compares the hop counts to a destination and selects the path with the smallest distance or hops. This algorithm is simple and does not require a powerful router or a lot of memory. RIP does not take into account the available bandwidth in best path determination. OSPF selects a path using cost, a metric based on bandwidth. All OSPF routers must obtain complete information about the networks of every router to calculate the shortest path. This is a complex algorithm. Therefore, OSPF requires more powerful routers and more memory than RIP. RIP uses a flat topology. Routers in a RIP region exchange information with all routers. OSPF uses the concept of areas. A network can be subdivided into groups of routers. In this way OSPF can limit traffic to these areas. Changes in one area do not affect performance in other areas. This hierarchical approach allows a network to scale efficiently. Interactive Media Activity Checkbox: Link-State and Distance Vector Comparison When the student has completed this activity, the student will be able to identify the difference between link-state and distance vector routing protocols. Web Links OSPF versus RIP http://www.cisco.com/warp/public/ 104/2.html#1.1
Content 2.2 Single Area OSPF Concepts 2.2.4 Shortest path algorithm The shortest path algorithm is used by OSPF to determine the best path to a destination. In this algorithm, the best path is the lowest cost path. The algorithm was discovered by Dijkstra, a Dutch computer scientist, and was explained in 1959. The algorithm considers a network to be a set of nodes connected by point-to-point links. Each link has a cost. Each node has a name. Each node has a complete database of all the links and so complete information about the physical topology is known. All router link-state databases are identical. The table in Figure shows the information that node D has received. For example, D received information that it was connected to node C with a link cost of 4 and to node E with a link cost of 1. The shortest path algorithm then calculates a loop-free topology using the node as the starting point and examining in turn information it has about adjacent nodes. In Figure , node B has calculated the best path to D. The best path to D is by way of node E, which has a cost of 4. This information is converted to a route entry in B which will forward traffic to C. Packets to D from B will flow B to C to E, then to D in this OSPF network. In the example, node B determined that to get to node F the shortest path has a cost of 5, via node C. All other possible topologies will either have loops or a higher cost paths. Web Links Shortest Path Algorithm http://www.cisco.com/warp/public/ 104/2.html#2.0
Content 2.2 Single Area OSPF Concepts 2.2.5 OSPF network types A neighbor relationship is required for OSPF routers to share routing information. A router will try to become adjacent, or neighbor, to at least one other router on each IP network to which it is