Overview
OSPF is an interior gateway protocol (IGP) that runs within a single autonomous system (AS). Exterior gateway protocols (EGP), such as Border Gateway Protocol (BGP), exchange routing information between ASs.
OSPF is a link state routing protocol, similar to the Intermediate System-to-Intermediate System (IS-IS) routing protocol. It advertises the states of its local network links. This distinguishes OSPF from some IGPs such as Routing Information Protocol (RIP). A distance vector protocol, such as RIP, advertises the distances (that is, the number of hops) to each known destination within the network.
Each participating OSPF router within the AS has an identical database describing the AS's topology. Each individual piece of this database is a particular router's local state. From this database, OSPF calculates a routing table by constructing a shortest-path tree.
OSPF learns the best routes to reachable destinations. It can quickly perceive changes in the topology of an AS and, after a short convergence period, calculate new loop-free routes. This protocol has been designed expressly for the TCP/IP Internet environment, including explicit support for classless interdomain routing (CIDR) and the tagging of externally derived routing information.
This chapter provides direction for customizing basic OSPF settings if you need to do so. For detailed information on the OSPF commands, see the ERX Command Reference Guide N to Z.
Terms
Table 8-1 defines commonly used OSPF terms.
Table 8-1 OSPF-related terms
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The relationship between selected neighboring routers for exchanging routing information. Not every pair of neighboring routers is adjacent. |
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An OSPF area is a collection of network segments interconnected by routers. It is a region in an OSPF routing domain. |
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A router that sits on the edge of an OSPF area and routes link state advertisements (LSAs) between areas. |
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A unique number that identifies an area. Typically, an IP address. |
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Contains verification information and is 64-bits in length. |
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A simple password is an example. All OSPF protocol exchanges are authenticated. The authentication type is configurable on a per-area basis. |
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A group of routers using a common routing protocol to exchange routing information. |
autonomous system boundary router (ASBR) |
An OSPF router that redistributes routes from other autonomous systems. |
classless interdomain routing (CIDR) |
CIDR replaces the traditional class structure of IP addresses. In CIDR, an IP network is represented by a prefix and a notation that indicate the IP address and mask—for example, 10.12.8.3/16 |
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Generates an LSA for the network. Performs other special responsibilities in running the protocol. |
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A collection of routers that use a common IGP constitutes an OSPF routing domain. |
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The distribution and synchronization of the link-state database between OSPF routers. |
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Establishes and maintains neighbor relationships. Also, dynamically discovers neighboring routers on broadcast networks. |
interior gateway protocol (IGP) |
A routing protocol that routers within an AS use to exchange information. |
link state advertisement (LSA) |
A unit of data that describes the local state of a router or network. LSAs are flooded throughout the routing domain. |
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Routers that have interfaces to a common network. |
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A network that has no broadcast capability but supports more than two routers. |
Not-so-stubby area (NSSA) |
Similar to a stub area but can also import selected external LSAs. |
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A 32-bit number that uniquely identifies a router within an AS—for example, 10.10.1.5 |
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An area that does not get flooded with external LSAs but does carry intra-area and interarea routes and a default route. |
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A logical link between two backbone routers where the link tunnels through a nonbackbone area. |
Figure 8-1 illustrates the topology of an OSPF routing domain.
