Chapter 28 OSPF
XGS-4528F User’s Guide
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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 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).
28.1.3 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.
28.1.4 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 to become DR or BDR. Only router A should become the DR.