OSPF is a routing
protocol for IP. It is a link-state protocol, as opposed to a distance-vector
protocol. A link-state protocol makes its routing decisions based on the states
of the links that connect source and destination machines. The state of the
link is a description of that interface and its relationship to its neighboring
networking devices. The interface information includes the IP address of the
interface, network mask, type of network to which it is connected, routers
connected to that network, and so on. This information is propagated in various
types of link-state advertisements (LSAs).
A router stores the
collection of received LSA data in a link-state database. This database
includes LSA data for the links of the router. The contents of the database,
when subjected to the Dijkstra algorithm, extract data to create an OSPF
routing table. The difference between the database and the routing table is
that the database contains a complete collection of raw data; the routing table
contains a list of shortest paths to known destinations through specific router
OSPF is the IGP of
choice because it scales to large networks. It uses areas to partition the
network into more manageable sizes and to introduce hierarchy in the network. A
router is attached to one or more areas in a network. All of the networking
devices in an area maintain the same complete database information about the
link states in their area only. They do not know about all link states in the
network. The agreement of the database information among the routers in the
area is called convergence.
At the intradomain
level, OSPF can import routes learned using Intermediate System-to-Intermediate
System (IS-IS). OSPF routes can also be exported into IS-IS. At the interdomain
level, OSPF can import routes learned using Border Gateway Protocol (BGP). OSPF
routes can be exported into BGP.
Information Protocol (RIP), OSPF does not provide periodic routing updates. On
becoming neighbors, OSPF routers establish an adjacency by exchanging and
synchronizing their databases. After that, only changed routing information is
propagated. Every router in an area advertises the costs and states of its
links, sending this information in an LSA. This state information is sent to
all OSPF neighbors one hop away. All the OSPF neighbors, in turn, send the
state information unchanged. This flooding process continues until all devices
in the area have the same link-state database.
To determine the best
route to a destination, the software sums all of the costs of the links in a
route to a destination. After each router has received routing information from
the other networking devices, it runs the shortest path first (SPF) algorithm
to calculate the best path to each destination network in the database.
The networking devices
running OSPF detect topological changes in the network, flood link-state
updates to neighbors, and quickly converge on a new view of the topology. Each
OSPF router in the network soon has the same topological view again. OSPF
allows multiple equal-cost paths to the same destination. Since all link-state
information is flooded and used in the SPF calculation, multiple equal cost
paths can be computed and used for routing.
On broadcast and
nonbroadcast multiaccess (NBMA) networks, the designated router (DR) or backup
DR performs the LSA flooding.
OSPF runs directly on
top of IP; it does not use TCP or User Datagram Protocol (UDP). OSPF performs
its own error correction by means of checksums in its packet header and LSAs.
In OSPFv3, the
fundamental concepts are the same as OSPF Version 2, except that support is
added for the increased address size of IPv6. New LSA types are created to
carry IPv6 addresses and prefixes, and the protocol runs on an individual link
basis rather than on an individual IP-subnet basis.
requires coordination among many internal routers: Area Border Routers (ABRs),
which are routers attached to multiple areas, and Autonomous System Border
Routers (ASBRs) that export reroutes from other sources (for example, IS-IS,
BGP, or static routes) into the OSPF topology. At a minimum, OSPF-based routers
or access servers can be configured with all default parameter values, no
authentication, and interfaces assigned to areas. If you intend to customize
your environment, you must ensure coordinated configurations of all routers.