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summaries, local changes do not have to be propagated to other area routers. This
allows OSPF to be easily scaled for larger networks, and provides a more stable
network topology.
Command Usage
• Use the Area Range Configuration page to summarize intra-area routes, and
advertise this information to other areas through Area Border Routers (ABRs). The
summary route for an area is defined by an IP address and network mask. You
therefore need to structure each area with a contiguous set of addresses so that
all routes in the area fall within an easily specified range. If it is not possible to use
one contiguous set of addresses, then the routes can be summarized for several
area ranges.This router also supports Variable Length Subnet Masks (VLSMs), so
you can summarize an address range on any bit boundary in a network address.
• To summarize the external LSAs imported into your autonomous system (i.e., local
routing domain), use the Summary Address Configuration screen (page 20-33).
Command Attributes
• Area ID – Identifies an area for which the routes are summarized. (The area
ID must be in the form of an IPv4 address.)
• Range Network – Base address for the routes to summarize.
• Range Netmask – Network mask for the summary route.
• Advertising – Indicates whether or not to advertise the summary route. If the
routes are set to be advertised, the router will issue a Type 3 summary LSA for
each specified address range. If the summary is not advertised, the specified
routes remain hidden from the rest of the network. (Default: Advertise)
20-23
Configuring the Open Shortest Path First Protocol
Console#show ip ospf
Routing Process with ID 192.168.1.253
Supports only single TOS(TOS0) route
Number of area in this router is 3
Area 0.0.0.0 (BACKBONE)
Number of interfaces in this area is 1
SPF algorithm executed 40 times
Area 0.0.0.2 (STUB)
Number of interfaces in this area is 1
SPF algorithm executed 8 times
Area 0.0.0.3 (NSSA)
Number of interfaces in this area is 1
SPF algorithm executed 40 times
Console#
20
4
2
-
3
9
Configuring Area Ranges
(Route Summarization for ABRs)
A
n OSPF area can include a large number of nodes.
If the Area Border Router (ABR) has to advertise
route information for each of these nodes, this
wastes a lot of bandwidth and processor time.
area area
A
BR
Instead, you can configure an ABR to advertise a summary
route
single summary route that covers all the individual
networks within its area. When using route