How Ospf Works; Interfaces And Virtual Links; Configuring Ospf; Figure 76 Ospf Network Example - ZyXEL Communications GS-4012F/4024 User Manual

GS-4012F/4024 User's Guide
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 76 OSPF Network Example

25.1.2 How OSPF Works

Layer 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.

25.1.3 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 ink must be configured on both layer 3 devices in the non-
backbone area and the backbone.

25.1.4 Configuring OSPF

To configure OSPF on the switch, do the following tasks
1 Enable OSPF
2 Create OSPF areas
3 Create and associate interface(s) to an area
4 Create virtual links to maintain backbone connectivity.
161 Chapter 25 OSPF