Discussion

Chapter
CHAPTER
12
12
OSPF 12
12.0 Introduction
The Open Shortest Path First (OSPF) protocol is an IGP that routes packets within a
single AS, or domain. The IETF began work on OSPF in the late 1980s to develop a
replacement for RIP, which was the only routing protocol at the time, because people
felt that a stronger routing protocol was needed and the link-state algorithm
looked promising. OSPF was implemented by router vendors in the early 1990s and
was eventually standardized by the IETF in 1997 as OSPF Version 1. The current
standard is Version 2, defined in RFC 2328. Much of the OSPF design was lifted
from IS-IS, which is an ISO routing-protocol standard developed at the same time.
OSPF was designed specifically for TCP/IP and explicitly supports IP subnetting and
the tagging of externally derived routing information. OSPF also provides for the
authentication of routing updates. RFC 2740 defines OSPF for IPv6.
As an IGP, OSPF works within a domain, which usually corresponds to an administrative
boundary and focuses on determining the most efficient routes to destinations
within a domain. EGPs, on the other hand, primarily focus on policy rather
than on the most efficient routing.
OSPF is a link-state protocol and uses link-state advertisements (LSAs) to describe
the network topology. Each OSPF router generates LSAs that describe the topology it
sees and floods the LSAs throughout the domain. As a result, each router ends up
with a link-state database that describes the same network topology. Once the router
has the complete network topology, it runs the Dijkstra SPF calculation to determine
the shortest path to each destination in the network. The calculation results in
destination/next-hop pairs that are placed in the OSPF routing database. Each router
performs the SPF calculation independently, and the result is that each OSPF router
has an identical routing database (though each router has different next hops for the
destinations).
OSPF runs directly over IP, using IP protocol 89. It does not use a transport layer
protocol such as TCP or UDP.
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