On an OSPF network, an algorithm is used to prevent loops within an area, but loops may occur between areas. Therefore, OSPF defines a loop prevention mechanism for inter-area routes.
Which of the following statements are true about the loop prevention mechanism?
- Inter-area routes cannot be directly transmitted between non-backbone areas.
- All non-backbone areas must be directly connected to area 0.
- Inter-area routes need to be forwarded through area 0.
- An ABR cannot inject Type 3 LSAs that describe routes to a network segment in an area back to the same area.
Answer(s): A,B,C,D
Explanation:
OSPF Area Design and Loop Prevention:
OSPF uses a hierarchical structure with areas to improve scalability and efficiency. Area 0, the backbone area, plays a crucial role in ensuring loop-free route distribution between areas. The following mechanisms are key to preventing routing loops:
Strict adherence to hierarchical area design.
Prohibition of direct inter-area route exchanges between non-backbone areas.
Reference:
HCIP-Datacom-Core Technology Training Material (OSPF Basics and Advanced Concepts).
Analysis of Each Statement:
A . Inter-area routes cannot be directly transmitted between non-backbone areas. This statement is TRUE. OSPF mandates that all inter-area routing must pass through Area 0. Direct inter-area route exchanges between two non-backbone areas are not allowed to prevent loops.
HCIP-Datacom-Core Technology Training Material (Inter-Area Routing Mechanisms).
B . All non-backbone areas must be directly connected to area 0. This statement is TRUE. OSPF requires every non-backbone area to connect directly to Area 0 to facilitate loop-free inter-area routing. Virtual links may be configured in exceptional cases where direct connection is not possible.
HCIP-Datacom-Core Technology Training Material (OSPF Backbone and Area Connectivity).
C . Inter-area routes need to be forwarded through area 0. This statement is TRUE. All inter-area traffic must traverse Area 0 to ensure hierarchical routing and loop prevention. This rule is a core design principle of OSPF.
HCIP-Datacom-Core Technology Training Material (Routing Control and Loop Prevention in OSPF).
D . An ABR cannot inject Type 3 LSAs that describe routes to a network segment in an area back to the same area.
This statement is TRUE. OSPF explicitly prohibits an ABR from injecting Type 3 LSAs describing a route into the same area where the route originates. This mechanism prevents routing loops within an area.
HCIP-Datacom Advanced Routing & Switching Technology (OSPF LSA Types and ABR Behavior).
Conclusion:
All options (A, B, C, D) are correct. OSPF enforces a robust loop prevention mechanism through hierarchical routing, mandatory traversal via Area 0, and strict rules on LSA propagation by ABRs. This ensures reliable and loop-free inter-area routing in OSPF networks.