An OSPF stub area is a special area type designed to reduce the size of the link-state database (LSDB) and routing tables on internal routers. This is achieved by preventing the flooding of Type 5 LSAs (AS-External-LSAs), which advertise routes external to the OSPF Autonomous System (AS). Instead of numerous external routes, the Area Border Router (ABR) connected to the stub area injects a single default route (via a Type 3 Summary LSA) into the area. This allows routers within the stub area to reach external destinations without needing to maintain information about each specific external prefix.

A. Type 1 LSAs (Router LSAs) are fundamental for describing router links within an area and are not restricted in any area type.

B. Type 2 LSAs (Network LSAs) describe multi-access network segments within an area and are essential for intra-area routing.

C. Type 3 LSAs (Summary LSAs) are used by ABRs to advertise routes from other areas and are permitted in stub areas to ensure inter-area reachability.

1. IETF RFC 2328 - OSPF Version 2:

Section 3.6

"Stub areas": "Stub areas are areas that do not import AS-external-LSAs. Stub areas are protected from the complexities of external routing. Instead

a default route (IP destination 0.0.0.0) is used to handle all traffic to external destinations." This directly confirms that Type 5 LSAs (AS-external-LSAs) are blocked.

Section 12.4.5

"AS-external-LSAs": Defines these as Type 5 LSAs

which "describe routes to destinations external to the Autonomous System."

2. Kurose

J. F.

& Ross

K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson.

Chapter 5.3

"Routing in the Internet": In the discussion of OSPF

the text explains that to reduce overhead

an OSPF autonomous system can be configured into areas

and a "stub area" is defined as an area that does not receive link-state advertisements for routes external to the AS. This refers to the filtering of Type 5 LSAs.

3. Doyle

J.

& Carroll

J. (2006). Routing TCP/IP

Volume 1 (2nd ed.). Cisco Press.

Chapter 8

"OSPF

" Section "Stub Areas": "A stub area is an area into which AS External LSAs (Type 5) are not flooded... By filtering Type 5 LSAs

the LSDBs of internal routers are significantly reduced." This explicitly states the restriction of Type 5 LSAs.