Ospf Cost - D-Link DGS-3324SRi User Manual

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.
When the link-state database of each router is updated, the individual routers will calculate a Shortest Path Tree to
all destinations
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 tables written
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 Stackable Gigabit Layer 3 Switch Manual
with the individual router as the root. The IP routing table will then be made up of the
Figure 7- 1. Constructing a Shortest Path Tree
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