Installing and Administering Internet Services

342 Chapter8
Configuring gated
Configuring the OSPF Protocol
state advertisements used by the OSPF protocol.
AS boundary routers exchange routing information with routers in other
autonomous systems. An AS boundary router may be an area border
router or an internal router. It can be a backbone router, but it is not
required that an AS boundary router be a backbone router. An AS
boundary router learns about routes outside of its attached AS through
exchanges with other routing protocols or through configuration
information. Each AS boundary router calculates paths to destinations
outside of its attached AS. It then advertises these paths to all routers in
its AS.
There are two levels of routing in the AS:
• Intra-area routing, where the source and destination of a packet
both reside in the same area. Routing is handled by internal routers.
• Inter-area routing, where the source and destination of a packet
reside in different areas. Packets travel an intra-area route from the
source to an area border router, then travel an inter-area route on a
backbone path between areas, then finally travel another intra-area
route to the destination.
Planning Your OSPF Configuration
The following is a suggested sequence of steps in planning for OSPF
routing in your autonomous system:
Table 8-2 Types of Link State Advertisements
Type Content Originated By Flooded
Throughout
Router Link Router’s links to area Internal and area border
routers
Area
Network Link List of routers attached to
network
Designated Router Area
Summary link Routes to destination outside
area but within AS
Area border router Area
AS external link Routes to destinations outside
AS
AS boundary router AS