As networks grow in size the routing tables on each router grows as well. As a result the overhead to process the routes can grow exponentially and take resources away from other functions the router can be performing. As well, large routing tables can be difficult to read quickly, efficiently and troubleshoot. You can make your life extremely easy if you engineer your IP addressing using an harmonious numbering system and avoid using a discontinuous network address scheme. If you adhere to these methods and use CIDR addresses you can also take advantage of Route Summarization (aka Route Aggregation)

An example of a route summariztion would be: If you have an ASBR that manages five 24bit networks.

172.16.0.1/24
172.16.1.1/24
172.16.2.1/24
172.16.3.1/24
172.16.4.1/24

In a neighbouring router’s table, these networks will be listed as attached networks with a next hop and value. And each route will be advertised to the LSDB. We can reduce the amount of advertisements by summerizing these 5 networks on the ASBR as one large Network so the neighbouring router will only receive one advertisement. This is accomplished by using a CIDR mask length that will encompass the entire range of the 5 example networks.

The five networks listed above consist of the range 172.16.0.1 – 172.16.4.255. This can be summarized as 172.16.0.0/21 or 172.16.0.0/255.255.248.0

To accomplish this you will need to understand CIDR, VLSM, IP, and binary Math.  The best site I have found that explains the math behind the calculations is.  http://subnettingmadeeasy.blogspot.com/2007/11/router-summarization.html note: the author of this article makes some great observations on how summarizing your routes can cause issues. He also describes some great work arounds.

or if you are like me. You can use an online calculator. The best one I have found is http://buchananweb.co.uk/security51.aspx

An ERS8600 CLI example of the route summary described above would look like this.

ip ospf admin-state enable
ip ospf router-id 172.16.0.0
ip ospf enable
ip ospf area 0.0.0.1 create
ip ospf area 0.0.0.1 range 172.16.0.0/255.255.248.0 create advertise-mode summarize lsa-type summary-link advertise-metric 10