Ospf Cost - D-Link xStack DES-3800 Series User Manual

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:
When OSPF is started, or when a change in the routing information changes, the router
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generates a link-state advertisement. This advertisement is a specially formatted packet that
contains information about all the link-states on the router.
This link-state advertisement is flooded to all router in the area. Each router that receives the
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link-state advertisement will store the advertisement and then forward a copy to other routers.
When the link-state database of each router is updated, the individual routers will calculate a
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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.
Once the link-state databases are updated, Shortest Path Trees calculated, and the IP routing
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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.
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 calculates the shortest path to each destination based on the cumulative cost
to reach that destination over 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 will
have and use the exact same link-state database.
The following sections describe the information used to build the Shortest Path Tree.

OSPF Cost

Each OSPF interface has an associated cost (also called "metric") 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 interface has a lower cost). There is then a higher
cost (and longer time delays) in sending packets over a 56 Kbps dial-up connection than over a 10
Mbps Ethernet connection. The formula used to calculate the OSPF cost is as follows:
Cost = 100,000,000 / bandwidth in bps
As an example, the cost of a 10 Mbps Ethernet line will be 10 and the cost to cross a 1.544 Mbps T1
line will be 64.
Shortest Path Tree
To build Router A's shortest path tree for the network diagramed below, Router A is put at the root of
the tree and the smallest cost link to each destination network is calculated.
xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
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