The concept of route summarization is key in implementing a stable and
scalable OSPF internetwork. Route summarization is the consolidating of
advertised addresses by area border routers (ABRs). Instead of advertising
routes to individual nodes within an area, you can configure an ABR to
advertise a single summary route or "network range" that covers all the
individual networks within its area that fall into the specified range.
Route summarization helps to control routing table sizes and prevents
route flapping — that is, the constant changing of routes whenever an
interface within an area comes online or goes offline. Using route
summarization, routes within an area that change do not need to be
changed in backbone ABRs and other area routers.
For optimal route summarization of an area, structure the area with a
contiguous addressing scheme so that all routes within the area fall
within a specified address range. This summary route or address range is
defined by an IP address and mask combination. OSPF supports Variable
Length Subnet Masks (VLSMs), so you can summarize a range of
addresses on any bit boundary in a network or subnetwork address.
For example, an address range specified with an IP address of
188.8.131.52 with a mask of 255.255.0.0 describes a single route to a
collection of destinations between 184.108.40.206 and 220.127.116.11.
Consider the following guidelines when you design and configure areas:
Define individual areas as contiguous, that is, any router that
participates in OSPF exchanges must have a direct path to all other
OSPF routers in the same area.
Do not disconnect an area border router from the backbone area. This
action may result in a loss of network topology information and
Define redundant links between area routers to help prevent area
If a portion of your internetwork consists of a particularly high number
of nodes, consider creating an area specifically for that densely
connected portion of your network.