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Инструкция по эксплуатации Страница: 145

D-Link DGS-3312SR [145/290] Introduction to ospf

DGS-3312SR Stackable Gigabit Layer 3 Switch

OSPF

Introduction to OSPF

The Open Shortest Path First (OSPF) routing protocol that uses a link-state algorithm to determine routes to network

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 generates 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 ro ting table will then be

made up of the destination address, associated cost, and the address of the xt hop to reach each

destination.

Once the link-state data ted, Shortest Path Trees calculated, and the IP routing tables

written − if there are no su ges in the OSPF network (such as a network link going down)

very little OSPF t

Shortest Path Algorithm

The Shortest Path to a destination is calculated using the Dijkstra algorithm. Each router is places at the root of a tree and

then ca test path to each d er multiple

possible routes. Each router will then have its own Shortest Path Tree (from the perspective of its location in the network

area) even though every router in the area w

The following sections describe the inform

OSPF Cost

Each OSPF int o tric”) that is representative of the overhead required to send

packets over that interface. This cost is inversely proportional to the bandwidth of the interface (i.e. a higher bandwidth

interfa ere is then a 56 Kbps dial-up

connection than over a 10 Mbps Ethernet c

Cost = 100,000,000 / bandwidt

As an he e 64.

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 network, etc. The collection of link-states are then collected in a link-state

database that is maintained by routers running OSP

OSPF specifies how routers will communicate to

4. bases are upda

bsequent chan

there is raffic.

lculates the shor estination based on the cumulative cost to reach that destination ov

ill have and use the exact same link-state database.

ation used to build the Shortest Path Tree.

erface has an associated c st (also called “me

ce has a lower cost). Th higher cost (and longer time delays) in sending packets over a

onnection. The formula used to calculate the OSPF cost is as follows:

h in bps

example, the cost of a 10 Mbps Et rnet line will be 10 and the cost to cross a 1.544 Mbps T1 line will b

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D-Link DGS-3312SR [145/290] Introduction to ospf

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