Which area should be made stub and what type of stube area it should be ? These are the two questions which network designers keeps asking themselves again and again. Right decision can add to your network performance whereas wrong decision can result in suboptimal routing and degrade your network performance. Lets try to understand what is a stub area and how they should be placed. There are three types of stub areas -
Stubby - External routes are not advertised into stub areas, nor can they be generated from stub areas; routers in these areas rely on the default route to reach all externals.
Not-so-stubby Areas (NSSAs) - External routes are not advertised into NSSA areas (unless they originate within the area), but they can be generated within the area.
Totally Stubby - Neither external nor internal routes are advertised into a totally stubby area; all routers rely on a default route to reach any destination outside the area. Looking at the network diagram below, which presents how the areas are set up, to see if any of them can be stubbed.
Now in context to the network diagram above, lets try to figure out the correct placement of the areas types -
Stubby - External routes are not advertised into stub areas, nor can they be generated from stub areas; routers in these areas rely on the default route to reach all externals.
Not-so-stubby Areas (NSSAs) - External routes are not advertised into NSSA areas (unless they originate within the area), but they can be generated within the area.
Totally Stubby - Neither external nor internal routes are advertised into a totally stubby area; all routers rely on a default route to reach any destination outside the area. Looking at the network diagram below, which presents how the areas are set up, to see if any of them can be stubbed.
Now in context to the network diagram above, lets try to figure out the correct placement of the areas types -
Totally Stubby Areas
Generally you wouldn't make an area totally stubby unless it had only one exit point. Generally you would hardly ding areas with only one exit point. Therefore, it would be used very frequently, except your training labs because most of the trainers are very comfortable with "Totally Stub Labs Scenarios"
Not-So-Stubby Areas
Not-so-stubby areas are generally used for areas that originate externals and don't need any information about the interior of the network. If you aren't originating any external routes into the network, you probably won't need any NSSAs either.
Stubby Areas
Depending on traffic flow, some of these areas might make good candidates for regular stubs. In each area, it depends on the amount of traffic destined to external hosts and whether optimum routing is important:
Area 0 - This area cannot be made into any type of a stub in an OSPF network.
Depending on traffic flow, some of these areas might make good candidates for regular stubs. In each area, it depends on the amount of traffic destined to external hosts and whether optimum routing is important:
Area 0 - This area cannot be made into any type of a stub in an OSPF network.
Area 1 - This area probably has a good deal of traffic to external links, although that's not certain. If it does, it should remain a normal area. The number of routers in the area (two) also influences you, here; it's small enough that flooding some externals into area 1 probably isn't going to be a problem.
Area 2 - This area probably has very little contact with outside networks. If there is some host or service that external hosts will need to contact, suboptimum routing isn't much of an issue because both paths to the DMZ area are two hops. This could be a stub area
Areas 3 and 4 - There could be a great deal of traffic to external services from these areas, but there isn't much of a chance of suboptimal routing from them to the DMZ area. These can be stub areas.