Routing – the advantage of an “area” in OSPF configuration

Like Mike said (in the comments) the network command will search for interfaces within that range and start advertising through it. It will also take the subnet of that interface and advertise it into OSPF. A cleaner way of looking at it that now a days instead of giving the network command you can go into an interface and add it to the OSPF proccess. This in turn does the same as the network command (find interface - advertise it)

If you say /24 or /21 doesn´t matter it's just a wider scope for OSPF to search for the interface.

Now back to your issue, the /21 won´t be advertised unless you have an interface as /21 and are advertising it in OSPF. What you could do is a static route (pointing to null0) on the ASA and redistribute it into OSPF (redistribute static subnets). Other methods like summary address are only available in ABR and ASBR (OSPF Border routers). Or you could, if applicable inject a default route in OSPF at the ASA.

The area part totally depends on your setup but if we are only talking OSPF within routers and ASA we can safely assume that one Area 0 will suffice. Try not to get mixed up with areas and networks here. Without going into to much detail and explinations of area and LSA just remember the golden rule, every area has to connect to the center (area 0), and area 0 can not be discontiguous.

Also remember if you want to redistribute iBGP into OSPF you need to allow it within BGP with "bgp redistribute-internal".

Based on your information i hope this is what you where looking for.

EDIT: OSPF area

OSPF Backbone

Why is area 0 the backbone area in OSPF? Why must all other areas connect to it?

This is explained very well in RFC 3509, Section 1.2¹:

1.2 Motivation

In OSPF domains the area topology is restricted so that there must be a backbone area (area 0) and all other areas must have either physical or virtual connections to the backbone. The reason for this star-like topology is that OSPF inter-area routing uses the distance-vector approach and a strict area hierarchy permits avoidance of the "counting to infinity" problem. OSPF prevents inter-area routing loops by implementing a split-horizon mechanism, allowing ABRs to inject into the backbone only Summary-LSAs derived from the intra-area routes, and limiting ABRs' SPF calculation to consider only Summary-LSAs in the backbone area's link-state database.

OSPF is usually considered a link-state protocol. What some people miss is that OSPF uses both link-state protocol and distance-vector protocol algorithms.

• Routes within the backbone, or a non-backbone area are computed as a link-state protocol does (ref Dijkstra's algorithm).
• When OSPF must carry non-backbone routes through the backbone, it uses some distance-vector behavior (i.e. parts of the Bellman Ford algorithm) to propagate Type3 LSA metrics into non-backbone areas.

Simple example of OSPF's distance-vector behavior:

<-- Area 5 --><-- Area 0 --><--           Area 4           -->

R5-----------R1-----------R2------------R3---------------------R4
     Cost 3      Cost 5        Cost 7            Cost 12

               LSA-->          LSA-->
               Type3 LSA       Type3 LSA
               {From R1}       {From R2}
               R5 cost is 3    R5 cost is 8

Consider what happens to a /32 Loopback route for R5.

1. R5 sends a Type1 LSA containing the /32 Loopback
2. R1 (Area 5 ABR), is connected to Area 0; it translates the Type1 LSA into a Type3 LSA with a cost of 3.
3. R2 (Area 4 ABR) receives R1's Type3 LSA (metric 3) and changes the metric to R5's Loopback, based on R2's cost to R1. Now R2's Type3 LSA for R5 has a cost of 8. This is the distance-vector behavior I mentioned above.

Requiring all non-backbone routes to go through the backbone is a loop-prevention mechanism.

Connecting non-backbone OSPF areas at an ABR

If 2 areas aren't connected through area 0 (discontiguous), how does OSPF behaving as a link state protocol increase the possibility of routing loops?

As we saw above, OSPF uses distance-vector behavior to send routes through the Area 0 backbone. Distance-vector protocols have well-known limits, such as the count-to-infinity problem. OSPF would be vulnerable to the same issues, if we didn't have boundaries on its behavior.

¹_{RFC 3509 describes Cisco IOS's ABR behavior}