HP 200 Series Services And Applications page 270

Routing with OSPF
Routing Improvements
Routers maintain detailed topological information about area(s) of which
they are members, and they maintain summary information about networks
in other areas. Area border routers (1.0.0.1 and 2.0.0.1 in figure 2) exchange
summary information about their own areas with other area border routers.
Network summary information received by an area border router is then
transmitted to routers in its attached area(s). The type of LSA used to send
network summaries is called a summary-links advertisement. Like
router-links advertisements, summary-links advertisements are also sent at
30-minute intervals and are triggered when a link state changes. A
summary-links advertisement includes the following for each link advertised:
IP network number or IP address of the link.
Router ID (set to that of the local area border router).
Subnet mask used on the link.
Metric or cost from the area border router to the destination
network.
Routers exchange LSAs to build and maintain their topological databases.
Topological Database
Routers in each area have identical routing databases. This topological
database describes which routers are connected to which networks.
Attributes of the networks and routers, such as subnet masks, metrics, etc.,
are also maintained in the database. Each router constructs its routing table
from the database. The routing table contains the shortest path to every
network the router can reach. The benefit of maintaining a topological
database is that when a change occurs to the network, new loop-free routes
can be computed quickly using minimal network resources. When a link
outage occurs, for example, each router that detects the change floods LSAs
that describe the change to all other routers in the network. Each router
then modifies its database and recomputes the shortest path to each
remaining network.
RIP, by comparison, maintains just the best route to any given network.
When a change to the network occurs, routers must exchange their entire
routing table with each neighboring router to relearn the best route to every
network.
3-76