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| 1.0.-
Introduction about OSPF |
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| OSPF version 2 as is
explained in RFC 2328 is a very difficult document to be read. The
document was originally written by J. Moy in April 1998. Here I'm
trying to make a brief summary of this document: |
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| OSPF is a link-state
(LS) routing
protocol. Each router running OSPF mantains a database describing the Autonomous System's (AS) topology. The database is referred to as the Link-State database
(LS-database). All
routers run the same algorithm in parallel. From the LS-database each router
constructs a tree of shortest path to the rest of the network with itself as a root. Each router
distributes its local state throughout the AS by flooding. |
| OSPF routes IP packets based
solely on the destination IP address; they are routed "as-is", then, they are
not encapsulated in any further protocol headers. When several equal-cost
routes to a destination exist, traffic is distributed equally among them.
The cost of a route is described by a simple dimensionless metric. |
| OSPF allows sets of networks
to be grouped together on areas. The topology of an area is hidden from the
rest of the AS. All OSPF protocol exchanges are authenticated. Externally
derived routing data (learned from an Exterior Gateway Protocol such as BGP)
is advertised throughout the AS. |
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OSPF runs better in
broadcast networks. When running in non-broadcast networks it uses one of two
modes. First mode is called non-broadcast
multi-access or NBMA; this mode simulates the operation of
OSPF on a broadcast
network. Second mode is called Point-to-Multipoint (P2MP) and treats the
non-broadcast network as a collection of point-to-point (P2P) links. |
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| Two routers that have
interfaces to a common network are called neighboring routers. These routers
form relationships between them called adjacencies. The adjacencies are
formed to exchange routing information. |
| The unit of information
describing the local state of a router is called a Link-State Advertisement
(LSA).
These advertisements are flooded throughout the routing domain. The flooding
is done using the Hello protocol. |
| Each broadcast or NBMA
network that has at least two routers has a Designated Router
(DR) that is
elected by the Hello protocol. This router enables a reduction in the number
of adjacencies required to run OSPF. |
| An AS's LS-database
describes a directed graph. Vertices of the graph are routers and networks.
A graph edge connects two routers when they are attached via a physical
point-to-point network. An edge connecting a router to a network indicates
that the router has an interface to the network. |
| Networks can be transit or
stub. Transit are those capable of carrying traffic that is neither locally
originated or destined. Stubs are blind networks. |
| NBMA mode is the most
efficient way to run OSPF over non-broadcast networks. Nevertheless, it
requires that all routers are able to communicate directly. When this is not
possible is better to run OSPF in Point-to-Multipoint (P2MP) mode. In this mode,
OSPF treats all router-to-router connections over the non-broadcast network
as if they were point-to-point links. No Designated Router is elected for
the network, nor is there an LSA generated for the network. |
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