An OSPF stub area is designed to reduce the size of the link-state database (LSDB) within an area by blocking Type 5 LSAs (AS-external routes). The Area Border Router (ABR) injects a default route into the stub area for external reachability.

A standard stub area can be converted into a totally stubby area by configuring the ABR to also block Type 3 LSAs (inter-area summary routes), further simplifying the LSDB. Additionally, OSPF virtual links, which are used to connect a disconnected area to the backbone, cannot be configured through a stub area because the transit area for a virtual link must have a complete routing table, which stub areas lack by design.

A. Stub areas can have an ASBR

This is incorrect. The primary function of a stub area is to block Type 5 LSAs, which are originated by an Autonomous System Boundary Router (ASBR). Therefore, an ASBR cannot exist within a standard stub area.

B. Stub areas can also be not-so-stubby areas.

This is incorrect. A "stub area" and a "not-so-stubby area" (NSSA) are mutually exclusive configurations. An NSSA is a specific area type that allows an ASBR while still blocking Type 5 LSAs from other areas.

1. Juniper Networks TechLibrary, OSPF Stub and Not-So-Stubby Areas. This document states, "You cannot configure a virtual link through a stub area." and "A stub area does not have an AS boundary router." This directly supports answer C and refutes answer A.

Source: Juniper Networks TechLibrary, Junos OS 23.4, OSPF Feature Guide for Routing Devices, "OSPF Stub and Not-So-Stubby Areas".

2. Juniper Networks TechLibrary, Example: Configuring an OSPF Totally Stubby Area. This configuration guide demonstrates the set protocols ospf area stub no-summaries command. The no-summaries option is what converts a standard stub area into a totally stubby area on the ABR, confirming that this conversion is a standard feature. This supports answer D.

Source: Juniper Networks TechLibrary, Junos OS 23.4, OSPF Feature Guide for Routing Devices, "Example: Configuring an OSPF Totally Stubby Area".

3. RFC 5340: OSPF for IPv6. This is the standard for OSPFv3.

Appendix C.3, Virtual links: "The transit area for a virtual link cannot be a stub area." This provides the standards-based reason for answer C.

Section 3.6, Stub areas: "OSPF stub areas are areas that do not allow AS-external-LSAs to be flooded into them." This implies that an ASBR, the source of AS-external-LSAs, cannot be part of the area, refuting answer A.

4. RFC 3101: The OSPF Not-So-Stubby Area (NSSA) Option. This RFC defines the NSSA, presenting it as a distinct alternative to the standard stub area defined in the main OSPF specification. This distinction confirms that an area cannot be both a stub and an NSSA simultaneously, refuting answer B.

Source: IETF RFC 3101, Section 1, "Introduction".