Content Overview Open Shortest Path First Protocol (OSPF) is one of the most commonly used IP routing protocols in both enterprise and service provider networks. It is an open-standard protocol based primarily on RFC 2328. OSPF offers benefits over historical distance vector routing protocols, such as Routing Information Protocol Version 1 (RIPv1), including rapid convergence, lower bandwidth utilization, variable-length subnet masking (VLSM) support, and increased scalability, since it can be deployed in a multiarea hierarchical design. However, OSPF is a fairly complex protocol made up of several protocol handshakes, database advertisements, and packet types, and requires mastery of a terminology and command set. This module describes how OSPF works and how to implement and verify OSPF operations. The lessons move from simple to more advanced configuration topics.

Content 3.1 Review of OSPF Fundamentals and Features 3.1.1 Link-State Routing Protocols The need to overcome limitations of distance vector routing protocols led to the development of link-state routing protocols. With a distance vector routing protocol, the routers make their routing decisions based on hop count and rely on information provided by directly connected neighbors. This is commonly referred to as “routing by rumor.” For example, routers using RIP do not have a full picture of the network topology and can only scale to a maximum of 15 hops. With a link-state routing protocol such as OSPF, each router has a full picture of the network topology, including bandwidth information for links to remote networks. Therefore, each router is able to independently select a loop-free and efficient pathway, based on cost, to reach every network in the area. This enables OSPF to make better routing decisions than distance vector routing protocols such as RIP. For example, consider the topology in Figure . When this network is configured using RIP, RIP chooses the path via the 64k links to forward traffic from router RTA to router RTE, since the routing decision is based solely on hop count. However, OSPF recognizes the faster links and forwards the traffic via the T3 links. OSPF is also a classless routing protocol, which means that it carries subnet mask information along with route information. Therefore, unlike RIPv1, OSPF supports multiple subnet masks for the same major network, known as variable-length subnet masking (VLSM). Web Links OSPF Support Page
http://www.cisco.com/en/US/tech/tk365/tk480/
tsd_technology_support_sub-protocol_home.html

Content 3.1 Review of OSPF Fundamentals and Features 3.1.2 Overview of OSPF Operations Link-state routing protocols have the following characteristics:

Respond quickly to network changes
Send triggered updates when a network change occurs
Send periodic updates, known as link-state refresh, at long intervals, such as every 30 minutes

Link-state routing protocols generate routing updates only when a change occurs in the network topology. When a link-state changes, the device that detected the change creates a link-state advertisement (LSA) concerning that link. The exchange of LSAs between neighbors report the state of routers and links, hence the term link-state. Link-state information must be synchronized between routers, which means the following:

LSAs are reliable; there is a method for acknowledging the delivery of LSAs.
LSAs are flooded throughout the area (or throughout the domain if there is only one area).
LSAs have a sequence number and a set lifetime so that each router recognizes that it has the most up-to-date version of the LSA.
LSAs are periodically refreshed to confirm topology information before the information ages out of the link-state database.

LSAs are propagated to all neighboring devices using the reserved class D multicast address 224.0.0.5. When a router receives an LSA, it updates its link-state database (LSDB). The LSDB is used to calculate the best paths through the network. Link-state routers find the best paths to a destination by applying Dijkstra’s algorithm, also known as the Shortest Path First (SPF) algorithm, against the LSDB to build the SPF tree. The best paths (shortest paths) are then selected from the SPF tree and placed in the routing table. Depending on the network topology, that router may then flood LSAs to all other OSPF routers in its area to ensure that all routing devices update their databases before updating routing tables to reflect the new topology. Only by reliably flooding link-state information can every router in the area or domain ensure that it has the latest, most accurate view of the network to make reliable routing decisions that are consistent with the decisions of other routers in the network. The memory resources that are needed to maintain the neighbor, topology, and routing tables represent one drawback to link-state protocols. Interactive Media Activity Check Box: RIPv1 versus OSPF Upon completion of this activity, the student will be able to compare routing issues between RIPv1 and OSPF.

Content 3.1 Review of OSPF Fundamentals and Features 3.1.3 OSPF Data Structures OSPF and Intermediate System-to-Intermediate System (IS-IS) are classified as link-state routing protocols because of the manner in which they distribute routing information and calculate the best routes. Link-state routing protocols collect LSAs from all other routers in the network or from within a defined area of the network. When link-state routing protocols have collected this information, each router independently calculates its shortest paths to all destinations in the network using the SPF algorithm. Incorrect information from any particular router is less likely to cause confusion, because each router maintains its own view of the network. Each router in the network must keep a record of the following information so that they are all making consistent routing decisions:

Immediate neighbor routers: If the router loses contact with a neighboring router, it invalidates all paths through that router within a few seconds and recalculates its paths through the network. Neighbor adjacency information is stored in the neighbor table, also known as an adjacency database.
All routers in the network, or in its area of the network, and their attached networks: The router recognizes other routers and networks through LSAs, which are flooded through the network. LSAs are stored in a topology table, also called an LSDB.
Best paths to each destination: Each router independently calculates the best paths using the SPF algorithm. The best paths are then offered to the routing table or forwarding database. Packets arriving at the router are forwarded based on the information held in the routing table.

Content 3.1 Review of OSPF Fundamentals and Features 3.1.4 OSPF Adjacency Database A router running a link-state routing protocol must first establish adjacencies with its neighboring routers. A router achieves neighbor adjacency by exchanging hello packets with the neighboring routers. In general, routers establish adjacencies as follows:

The router sends and receives hello packets to and from its neighboring routers. The destination address of the OSPF packets is the reserved