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Configuring Area Ranges (Route Summarization for ABRs)
An 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.
Instead, you can configure an ABR to advertise a
single summary route that covers all the individual
networks within its area. When using route
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 the routes for an area. 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. 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 3-252).
Command Attributes
• Area ID – Identifies an area for which the routes are summarized. (The area
ID must be in the form of an IP 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
summary is not sent, the routes remain hidden from the rest of the network.
(Default: Advertise)
Note:
This router supports up 64 summary routes for area ranges.
Console#show ip ospf 4-282
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#
areaarea
ABR
summary
route