summaries, local changes do not have to be propagated to other a rea route rs. This
allows OSPF to be easily scaled for larger networks, and provid es a mo re stable
network topology.
Command Usage
• Use the Area Range Configuration page to summari ze intra-are a routes, an d
advertise this information to other areas through Area Border Routers (ABRs). The
summary route for an area is defined by an IP add ress and ne twork mask. Yo u
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 possib le to us e
one contiguous set of addresses, then the routes ca n be sum marized fo r seve ral
area ranges.This router also supports Variable Length Subnet Masks (VLSMs), so
you can summarize an address range on any bit boun dary in a netw ork addre ss.
• To summarize the external LSAs imported into your autonomous system (i.e., local
routing domain), use the Summary Address Configuration screen (p age 20- 33).
Command Attributes
• Area ID – Identifies an area for which th e routes are summarize d. (The area
ID must be in the form of an IPv4 address.)
• Range Network – Base address for the routes t o summarize.
• Range Netmask – Network mask for the sum mary ro ute.
• Advertising – I ndicates whether or not to adver tise the sum mary route. If the
routes are set to be advertised, the router will issue a Type 3 sum mary LSA for
each specified address range. If the sum mary is not adver tised, the spe cified
routes remain hidden from the rest of the network. ( Default: Adv ertise)

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