Installing and Administering Internet Services

Chapter 8 275
Configuring gated
Configuring the OSPF Protocol
The lowest cost OSPF path between nodes A and D is therefore through
node B. However, if there were a link failure between node B and LAN 2,
packets would be rerouted through node C.
There are other places in the /etc/gated.conf file where cost can
optionally be defined:
• 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. For more
information, see “AS External Routes (AS Boundary Routers Only)”
on page 275.
• 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 into 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. ASE routes are
learned by AS boundary routers through another routing protocol, such
as EGP, or through configured routes. gated supports the use of route
information from other autonomous systems that use other routing
protocols, such as EGP. AS external link advertisements are sent by AS
boundary routers and are flooded throughout the AS (with the exception
of configured stub areas). A single AS external link advertisement is sent
for each external route that the AS boundary router has learned about.