Understand Open Shortest Path First (ospf) Design Guide

area area-id range address mask
Where the area-id is the area which contains networks to be summarized. The "address" and
"mask" specifies the range of addresses to be summarized in one range. This is an example of
summarization:
In this diagram, RTB isummarizes the range of subnets from 172.16.0.64 to 172.16.0.95 into one
range: 172.16.0.64 255.255.224.0. To achieve this, mask the first three left most bits of 64 with a
mask of 255.255.224.0.

---

In the same way, RTC generates the summary address 172.16.0.96 255.255.224.0 into the
backbone. Note that this summarization was successful because we have two distinct ranges of
subnets, 64-95 and 96-127.
It is difficult to summarize if the subnets between area 1 and area 2 overlapped. The backbone
area would receive summary ranges that overlap and routers in the middle would not know where
to send the traffic based on the summary address.
This is the relative configuration of RTB:
RTB#
router ospf 100
area 1 range 172.16.0.64 255.255.224.0
Prior to Cisco IOS® Software Release 12.1(6), it was recommended to manually configure, on the
ABR, a discard static route for the summary address to prevent possible routing loops. For the
summary route shown, use this command:
ip route 172.16.0.64 255.255.224.0 null0
In Cisco IOS® 12.1(6) and higher, the discard route is automatically generated by default. To
discard route, configure commands under 
router ospf
:
Either 
[no] discard-route internal
●
Or 
[no] discard-route external
●
Note about summary address metric calculation: 
RFC 1583
called to calculate the metric for
summary routes based on the minimum metric of the component paths available.
RFC 2178
(now obsoleted by 
RFC 2328
) changed the specified method to calculate metrics for
summary routes so the component of the summary with the maximum (or largest) cost would
determine the cost of the summary.
Prior to Cisco IOS® 12.0, Cisco was compliant with the then-current 
RFC 1583
. As of Cisco IOS®
12.0, Cisco changed the behavior of OSPF to be compliant with the new standard, 
RFC 2328
.
This situation created the possibility of sub-optimal routing if all of the ABRs in an area were not
upgraded to the new code at the same time.
In order to address this potential problem, a command has been added to the OSPF configuration
of Cisco IOS® that allows you to selectively disable compatibility with 
RFC 2328
.
The new configuration command is under