On an OSPF network, if two routers with the same router ID run in different areas and one of the routers is an ASBR, LSA flapping occurs.

The OSPF Router ID is a 32-bit value that must be unique across all routers within a single OSPF Autonomous System (AS). If two routers are configured with the same Router ID, the stability of the Link-State Database (LSDB) is compromised.

In the scenario described, one of the routers with the duplicate ID is an Autonomous System Boundary Router (ASBR). An ASBR is responsible for originating Type-5 Link-State Advertisements (AS-External-LSAs) to advertise external routes into the OSPF domain. A critical characteristic of Type-5 LSAs is that they are flooded unchanged throughout the entire OSPF AS, across all areas.

When the ASBR generates a Type-5 LSA, it uses its Router ID as the "Advertising Router" field. This LSA will eventually be flooded to the other router that shares the same Router ID. Upon receiving this LSA, the second router will detect a conflict: an LSA appears to be originated by itself but contains information it did not generate. This router will then attempt to "correct" the LSDB by originating its own version of the LSA, often with a higher sequence number, or by attempting to flush the conflicting LSA. The original ASBR will see this new LSA and repeat the process, leading to a continuous cycle of LSA updates and withdrawals. This persistent instability is known as LSA flapping, which causes constant recalculation of the routing table and network instability.

The "False" option is incorrect because the uniqueness of the Router ID is a fundamental prerequisite for a stable OSPF domain. The presence of an ASBR with a duplicate ID ensures that the conflict is not contained within an area but is propagated AS-wide through Type-5 LSAs, making LSA flapping inevitable.

1. Huawei VRP V800R019C00 Product Documentation

OSPF Configuration

Section: OSPF Fundamentals > Basic OSPF Concepts > Router ID

Content: "An OSPF router ID is a 32-bit unsigned integer that uniquely identifies a router in an AS. Every router running OSPF must have a unique router ID... If two routers have the same router ID

routes may be unreachable because of incorrect SPF calculation." This document establishes the core principle of Router ID uniqueness within the entire AS. The consequence of violating this principle is incorrect SPF calculation

which is caused by the LSDB instability (flapping).

2. Huawei HedEx Documentation for NE40E Routers

Troubleshooting - OSPF

Section: OSPF Troubleshooting > Common OSPF Faults > OSPF Neighbor Relationship Flapping and Other Faults

Content: The documentation details troubleshooting steps for OSPF issues. In the context of duplicate Router IDs

it explains: "If router IDs conflict

LSAs may be incorrectly deleted

leading to network-wide route calculation errors." This incorrect deletion and re-advertisement cycle is the mechanism of LSA flapping. The guide emphasizes checking for unique router IDs across the entire network as a primary troubleshooting step.

3. RFC 2328 - OSPF Version 2

Section: 12.4

"The Link State Database" and Appendix F

"OSPF MIB"

Content: The RFC specifies that an LSA is uniquely identified by its LS type

Link State ID

and the Advertising Router's ID. Section 13

"The SPF Algorithm

" describes how the database is built based on LSAs from unique advertising routers. The entire logic of OSPF relies on the Router ID being a unique identifier for the source of topological information. A duplicate ID breaks this fundamental assumption

leading to an inconsistent database. The scenario with an ASBR and Type-5 LSAs ensures this inconsistency is propagated globally within the AS.