Appendix F. OSPF (Open Shortest Path First)

The following topics are covered in this appendix:

• OSPF Areas

• OSPF Convergence

• OSPF Route Summarization

• OSPF Authentication

Open Shortest Path First (OSPF) is a routing protocol developed for Internet Protocol (IP) networks by the Interior Gateway Protocol (IGP) working group of the Internet Engineering Task Force (IETF). OSPF was created in the mid-1980s because RIP (Routing Information Protocol) was (and is) incapable of serving large, heterogeneous internetworks.

OSPF has two primary characteristics:

• The protocol is open, which means that its specification is in the public domain. The OSPF specification is based on Request For Comments (RFC) 1247.

• OSPF is based on the SPF algorithm. The SPF algorithm in turn is based on the Dijkstra Algorithm. The Dijkstra Algorithm enables route selection based on link state versus distance vectors.

OSPF is a link-state routing protocol that sends link-state advertisements (LSAs) to all routers within the same hierarchical area. Information on attached interfaces, metrics used, and other variables are included in these LSAs. As OSPF routers accumulate link-state information, they use the SPF algorithm to calculate the shortest path to each node.

OSPF features include least-cost routing, multipath routing, and load balancing. OSPF was derived from an early version of the IS-IS protocol.

NOTE

OSPF is based on the mathematical concept known as Graph Theory. Graph Theory is a branch of mathematics, called discrete math, focusing on the properties of a variety of graphs. A good start to learning about graph theory can be found at this web site: http://campus.northpark.edu/wicksBook/GraphTheory/Intro/

OSPF, and later OSPFv2, calculates routes based on the destination IP address found in IP datagram headers, with no provisions made for route calculation to non-IP destinations. OSPF was designed to detect quickly and adapt to changes in the network topology (autonomous system). OSPF routing decisions are based on the state of the router interconnecting links within the autonomous system. Each OSPF router maintains a database of network link states, including information regarding its usable interfaces, known-reachable neighbors, and link-state information.

Routing table updates, known as link-state advertisements (LSAs) are transmitted, or flooded, to all other neighbors within a router's area.

OSPF was introduced to overcome some of the limitations found with RIP and RIPv2, such as these:

• RIP and RIPv2 both have a limit of 15 hops. A RIP network that spans more than 15 hops (15 routers) is considered unreachable.

• RIP cannot handle Variable Length Subnet Masks (VLSM); however, RIPv2 can. Given the shortage of IP addresses and the flexibility VLSM gives in the efficient assignment of IP addresses, this is considered a major flaw.

• Periodic broadcasts of the full routing table consume a large amount of bandwidth. This is a major issue with large networks, especially on slow links and WAN clouds.

• RIP and RIPv2 both converge slower than OSPF. In large networks, convergence gets to be in the order of minutes. RIP routers will go through a period of a hold-down and garbage collection and will slowly time-out information that has not been received recently. This is inappropriate in large environments and could cause routing inconsistencies.

• RIP and RIPv2 have no concept of network delays and link costs. Routing decisions are based on hop counts. The path with the lowest hop count to the destination is always preferred even if the longer path has a better aggregate link bandwidth and slower delays.

• RIP and RIPv2 networks are flat networks. There is no concept of areas or boundaries. With the introduction of classless routing and the use of network aggregation and summarization, RIP networks struggle to provide a coherent networking infrastructure.

NOTE

Although RIPv2 supports address summarization with the use of VLSM, the concept of areas is not supported.

Link-state protocols, such as OSPF, provide for several networking features that enable a more robust and flexible internetworking environment. These OSPF-enabled features are as follows:

• There is no hop count limitation.

• VLSM support is very useful in IP address allocation.

• OSPF uses IP multicast to send link-state updates. This ensures less processing on routers that are not listening to OSPF packets.

• OSPF updates are "event triggered," sent only in the case of routing changes occurring within the network instead of periodically.

• OSPF allows for better load balancing.

• OSPF allows for a logical definition of networks in a hierarchical network structure where routers can be divided into areas. This will limit the explosion of link-state updates over the whole network. This also provides a mechanism for aggregating routes and cutting down on the unnecessary propagation of subnet information.

• OSPF allows for the transfer and tagging of external routes injected into an Autonomous System. This keeps track of external routes injected by exterior protocols such as BGP.