Ospf Cost - D-Link DES-3350SR User Manual

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:
•
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.
•
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 − 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 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.
D-Link DES-3350SR Standalone Layer 3 Switch
Figure 9 - 13. Constructing a Shortest Path Tree
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