Chapter 28 OSPF
The following figure depicts an OSPF network example. The backbone is area 0 with a backbone router. The internal routers are in area 1 and 2. The area border routers connect area 1 and 2 to the backbone.
Figure 119 OSPF Network Example
28.1.2 How OSPF WorksLayer 3 devices exchange routing information to build synchronized link state database within the same AS or area. They do this by exchanging Hello messages to confirm which neighbor (layer 3) devices exist and then they exchange database descriptions (DDs) to create the link state database. The link state database in constantly updated through LSAs (Link State Advertisements).
The link state database contains records of router IDs, their associated links and path costs. Each device can then use the link state database and Dijkstra algorithm to compute the least cost paths to network destinations.
28.1.3 Interfaces and Virtual LinksAn OSPF interface is a link between a layer 3 device and an OSPF network. An interface has state information, an IP address and subnet mask associated with it. When you configure an OSPF interface, you first set an interface to transmit OSPF traffic and add the interface to an area.
You can configure a virtual link to establish/maintain connectivity between a
The OSPF protocol provides for automatic election of Designated Router (DR) and Backup Designated Router (BDR) on network segments. The DR and BDR keep track of link state updates in their area and make sure LSAs are sent to the rest of the network.
In most cases the default DR/BDR election is fine, but in some situations it must be controlled. In the following figure only router A has direct connectivity with all the other routers on the network segment. Routers B and C do not have a direct connection with each other. Therefore they should not be allowed to become DR or BDR. Only router A should become the DR.
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