Route Summarization (Abrs): Advertising A Link To One Area To Routers In Another Area - HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual

IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring OSPF
Note
13-44
You must configure each device in the stub area with the area <area ID>
stub command. Otherwise, devices will not be able to achieve adjacency.
Even though routers in a stub area only handle intra-area routing, hosts can
still reach other areas. The ABR for the stub area injects summary LSAs into
the area. Each summary LSA advertises a connection through the ABR to a
range of networks in another area. Because sites often connect through a
single router only, stub routers do not need more information than this.
Similarly, the ABR distributes summary LSAs for networks in the stub area so
that hosts in different areas can reach them.
You can also configure an area as a total stub area. This area receives a default
route, but it does not receive the separate summary LSAs for different areas.
This option is usually sufficient for routers with one path for all external and
inter-area traffic. Enter this OSPF configuration command:
Syntax: area <area ID> stub no-summary
If you want area 1 to be a total stub area, enter:
ProCurve(config-ospf)# area 1 stub no-summary
Route Summarization (ABRs): Advertising a Link to One Area to
Routers in Another Area
Route summarization allows a router to communicate and store a route to an
entire range of subnets in a single entry. OSPF is a classless routing protocol,
which means that it can summarize routes to subnets and ranges of subnets
with any number of network bits. The route simply includes the network bits
that apply to all of the subnets and a length prefix that specifies which bits
these are.
Route summaries specify more networks in fewer entries the further they go
from the destination. This makes sense because routes are simply directions.
When you are in the Eastern Hemisphere, you move in the same direction to
reach the United States, Canada, or Mexico. It is not until you near North
America that the route changes according to the specific country for which
you are headed.
In the same way, an ABR can summarize a connection to an entire range of
networks in an area. It then advertises this summary link to internal routers
in another area so that these routers can forward inter-area traffic.