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Chapter 67 OSPF
XS3800-28 User’s Guide
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The internal routers are in area 1 and 2. The area border routers connect area 1 and 2 to the backbone.
Figure 398 OSPF Network Example
67.1.2 How OSPF Works
Layer-3 devices exchange routing information to build a synchronized link state database within the
same AS or area. 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.
Layer-3 devices build a synchronized link state database 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 is constantly updated through LSAs (Link State
Advertisements).
67.1.3 Interfaces and Virtual Links
An 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 non-backbone area and
the backbone. The virtual link must be configured on both layer-3 devices in the non-backbone area
and the backbone.
67.1.4 OSPF and Router Elections
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