Route summarization is a method of condensing multiple IP address routing table entries.
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This reduces memory usage on routers.
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It also reduces the amount of routing protocol traffic.
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To create an environment capable of supporting route summarization, you must implement an effective hierarchical addressing scheme.
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The addressing structure that you implement can have a profound effect on the performance and scalability of your OSPF internetwork.
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Route summarization in an OSPF internetwork occurs between each area and the backbone area.
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Route summarization must be configured manually in OSPF.

It is disabled by default in OSPF routers.
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You can assign addresses using many different options, such as
 
• unique network numbers for 
  each area
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• NIC-authorized address areas 
  created using bit-wise subnetting 
  and VLSM
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• private addressing with a buffer zone
  to the official Internet world
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In the first option you create a backbone and multiple areas and assign a separate network address for each.
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Be sure to remember to configure your area border routers for route summarization.
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The benefits of assigning addresses in this way are
 
• address assignment is 
  relatively easy to remember
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• configuration of routers is 
  relatively easy and mistakes 
  are less likely
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• network operations are 
  streamlined because each 
  area has a simple, unique 
  network number
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Route summarization at the area border routers is greatly simplified.
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The main drawback of this approach to address assignment is that it wastes address space.
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In assigning addresses for route summarization, you can use bit-wise subnetting and variable-length subnetwork masks (VLSMs) in combination to save address space.
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Here is an example of route summarization.
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The IA entry in the routing table
stands for Inter-Area summary route
and contains a mask indicating that
one entry can be used to represent a
large range of subnets, router A is an
internal router in area 2, router B is
the ABR and router C is the backbone 
router in area 0 (the backbone).
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Private addressing is another option when assigning addresses.
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This allows you to be as flexible and scalable as you want.
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If you develop a large-scale internetwork of privately addressed IP nodes you will have limited access to the Internet.
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You will have to implement what is referred to as a @Rdemilitarized zone@r (DMZ).
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A DMZ provides a buffer of nodes with public IP addresses between a privately addressed network and the Internet.
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All nodes on the network in the DMZ must have public-assigned IP addresses.
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For example, a DMZ might contain two routers and a gateway host.
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One router provides the interface between the DMZ and the Internet and the other provides the firewall between the DMZ and the private address environment.
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All applications that need to run over the Internet must access the Internet through the application gateway.
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Route summarization is particularly important to an OSPF environment because it increases the stability of the network.
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If route summarization is being used, routes that change within an area do not need to be changed in the backbone or in other areas.
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When you are setting up route summarization you should consider
 
• what information does 
  the backbone need to 
  know about each area?
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• what information does 
  each area need to know 
  about the backbone and 
  other areas?
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In setting up your OSPF areas bear in mind that
 
• OSPF route summarization 
  occurs in the ABRs
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• OSPF supports VLSM (so it 
  is possible to summarize on 
  any bit boundary in a 
  network or subnet address)
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• OSPF requires manual 
  route summarization
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There are four types of routing information that may need to be handled in an area:
 
• default routes

• intra-area routes

• inter area routes

• external routes
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If an explicit route to a given IP network or subnetwork cannot be found, the router will forward the packet to a destination specified by the default route.
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Each area must have explicit network or subnet routes for all networks or subnets within an area.
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These are called intra-area routes.
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Areas may also carry explicit network or subnet routes for networks or subnets that are in the AS but not in this area.
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These are known as inter area routes.
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When different ASs exchange routing information, the routes that exchange are referred to as external routes.
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In general, you should restrict the routing information in any area to the minimal set that the area needs.
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When you are designing an OPSF internetwork for efficient route selection you should consider three topics:
 
• tuning OSPF metrics
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• controlling inter area traffic
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• load balancing
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The default value for OSPF metrics is based on bandwidth.
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Each link in a route is given a metric value equal to the inverse of the bandwidth for that link.
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The metric for a route is the sum of the metrics for all the links in the route.
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When route summarization is enabled, OSPF uses the metric of the best route in the summary.
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Link metrics are normalized to give FDDI a metric value of 1.
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However, in some cases your network might implement a medium type that is faster than FDDI, for example ATM.
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By default a faster medium will be assigned a metric value equal to the metric value of an FDDI link.
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Given an environment with both FDDI and a faster medium type, you must manually configure link metric values so that the faster link has a lower value.
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You enter @Rip ospf cost@r at the interface prompt to modify the link metric value.
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There are two forms of external metrics - type 1 and type 2.
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If you use an external type 1 metric, route metrics are calculated by adding the internal route metric to the external route metric.
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Using type 2 you do not add the internal route metric to the external route metric.
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The external type 1 metric is generally preferred.
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It is important to control inter area traffic.
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When an area has only one ABR, all traffic that does not belong in the area is sent to that ABR.
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In areas that have multiple ABRs, two choices are available for traffic that needs to leave the area:
 
• use the ABR closest to the 
  originator of the traffic
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• use the ABR closest to the 
  destination of the traffic
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If the ABRs inject only the default route into the area, the traffic goes to the ABR that is closest to the source of the traffic.
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This means that the traffic leaves the area as soon as possible.
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This can be desirable because the backbone typically has higher bandwidth lines available.
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If you want the traffic to use the ABR that is nearest to the destination, the ABRs should inject summaries into the area instead of just injecting the default route.
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Doing this means that the traffic leaves the area as late as possible.
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Most network designers prefer to avoid asymmetric routing.
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Asymmetric routing means that the packets going from A to B use a different path than the packets going from B to A.
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In order to avoid asymmetric routing it is important to understand how routing occurs between areas.
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You should try to balance the traffic load in your OSPF internetwork.
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Internetwork topologies are generally designed to provide redundant routes in order to prevent a partitioned network.
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These redundant routes are also useful in providing additional bandwidth in high traffic areas.
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If equal-cost paths exist between nodes, Cisco routers automatically load balance in an OSPF environment.
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Cisco routers can use up to four equal-cost paths for a given destination.
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Packets can be distributed either based on destination or on a packet-by-packet basis.
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Distributing packets based on destination is the default load balancing method on Cisco routers.
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This is recommend for line speeds of 56 Kbps and faster.
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You can use the @Rno ip route-cache@r at the interface prompt to turn off the distribution based on destination.
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This allows packets to be distributed on a packet-by-packet basis.