OSPF

1. VL and area 0 range.

If an area is capable of carrying transit traffic (i.e., its TransitCapability is set to TRUE), routing
information concerning backbone networks should not be condensed before being summarized into the area. Nor should the advertisement of backbone networks into transit areas be suppressed. In other words, the backbone’s configured ranges should be ignored when originating summary-LSAs into transit areas.

To show if the area is transitCapability, do
R1# show ip ospf
=================
Area 1
Number of interfaces in this area is 1
This area has transit capability: Virtual Link Endpoint
Area has no authentication
SPF algorithm last executed 01:29:49.932 ago
SPF algorithm executed 13 times
Area ranges are
Number of LSA 12. Checksum Sum 0x052BF3
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
========================================

After you enable the virtual link, the "area 0 range" won't works (only for backbone area0).

To fix it, do not use virtual link, instead you can create a GRE tunnel and set it to area 0.

2. VL and GRE Tunnel
Virtual-link:
-It is considered part of (Area 0) by default, without any additional configuration.
-It dose not require any kind of addressing.
-Configuration is only needed under the OSPF routing process.
-Only routing updates are tunneled into the virtual-link, but data traffic is not.
-Transit area can not be a stub area.
GRE tunnel:
-Tunnel interfaces must be created and addressing is required. (can be unnumbered).
-Tunnel Address must be advertised into (Area 0) using a network command.
-Both routing updates and data traffic are tunneled; this introduces more overhead.
-Transit area can be any type; this means its your only option if the transit is a stub area.

3. Default route. Link.
You can not redistribute default route into ospf. It can be inserted into the OSPF domain only as an external or inter-area (summary) route. There are only two ways to generator default-route into ospf by:
a. default-information originate
b. summary only in stub or nssa. (you cannot do 'area 0 range 0.0.0.0 0.0.0.0)