KTI KGS-3120, Руководство по эксплуатации

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Introduction to OSPF
The Open Shortest Path First (OSPF) routing protocol that uses a link-state algorithm to determine
routes to network destinations. A [link] is an interface on a router and the [state] is a description of
that interface and its relationship to neighboring routers. The state contains information such as the IP
address, subnet mask, type of network the interface is attached to, other routers attached to the net-
work, etc. The collection of link-states are then collected in a link-state database that is maintained by
routers running OSPF.
OSPF specifies how routers will communicate to maintain their link-state database and defines several
concepts about the topology of networks that use OSPF.
To limit the extent of link-state update traffic between routers, OSPF defines the concept of Area. All
routers within an area share the exact same link-state database, and a change to this database on one
router triggers an update to the link-state database of all other routers in that area. Routers that have
interfaces connected to more than one area are called Border Routers and take the responsibility of
distributing routing information between areas.
One area is defined as Area 0 or the Backbone. This area is central to the rest of the network in that
all other areas have a connection (through a router) to the backbone. Only routers have connections to
the backbone and OSPF is structured such that routing information changes in other areas will be
introduced into the backbone, and then propagated to the rest of the network.
When constructing a network to use OSPF, it is generally advisable to begin with the backbone (area 0)
and work outward.
Link-State Algorithm
An OSPF router uses a link-state algorithm to build a shortest path tree to all destinations known to the
router. The following is a simplified description of the algorithm
’
s steps:
1. When OSPF is started, or when a change in the routing information changes, the router gener-
ates a link-state advertisement. This advertisement is a specially formatted packet that contains
information about all the link-states on the router.
2. This link-state advertisement is flooded to all router in the area. Each router that receives the
link-state advertisement will store the advertisement and then forward a copy to other routers.
3. When the link-state database of each router is updated, the individual routers will calculate a
Shortest Path Tree to all destinations - with the individual router as the root. The IP routing table
will then be made up of the destination address, associated cost, and the address of the next hop
to reach each destination.
4. Once the link-state databases are updated, Shortest Path Trees calculated, and the IP routing
tables written - if there are no subsequent changes in the OSPF network (such as a network link
going down) there is very little OSPF traffic.