Configuring gated

Configuring the OSPF Protocol

In Figure 3-12, there are two possible packet routes between nodes A and

D:one route goes through node B and the other route goes through node C. The cost of each route is calculated as follows:

Node A to node B and node B to node D: 5+5 = 10

Node A to node C and node C to node D: 5+10 = 15

The lowest cost OSPF path between nodes A and D is therefore through node B. However, packets are rerouted through node C if there is a link failure between node B and LAN 2.

You can optionally define cost in the following places in the /etc/gated.conf file:

•In a defaults statement in the OSPF protocol configuration, which applies only to AS boundary routers. This cost definition applies to routes to destinations outside the AS. These routes may have been derived from other routing protocols, such as EGP. See “AS External Routes (AS Boundary Routers Only)” on page 80 for more information.

•In the export statement in the Control class in the /etc/gated.conf file, which applies only to AS boundary routers. This cost definition applies to routes that are exported from the AS boundary router to routers in other autonomous systems.

•In the stub area definition of the OSPF protocol configuration. This cost definition specifies the cost of the default summary link that is advertised in the area.

•In the stubhosts definition of the OSPF protocol configuration. This cost definition specifies the cost of a point-to-point interface between the router and a non-OSPF host.

•In the subhosts definition of the OSPF protocol configuration. This cost definition specifies the cost of a point-to-point interface between the backbone router and a non-OSPF host.

AS External Routes (AS Boundary Routers Only)

AS external (ASE) routes are paths to destinations that are outside the AS. Most ASE routes are routes to specific destinations. AS boundary routers specify the ASE routes through another routing protocol, such as EGP, or through configured routes.

80

Chapter 3