Which statement correctly describes OSPF summarization?
- With route summarization, only the summarized routes are propagated into the stub areas.
- With route summarization, only the default route is propagated into the backbone (Area 0).
- With route summarization, all subnet routes are propagated into the backbone (Area 0).
- With route summarization, only the summarized routes are propagated into the backbone (Area 0).
Explanation: OSPF route summarization is an optimization technique where an Area Border Router (ABR) or Autonomous System Boundary Router (ASBR) consolidates multiple specific network prefixes into a single summary advertisement.
According to the sources, without route summarization, every individual specific-link LSA is propagated into the OSPF backbone and throughout the entire domain, which increases network traffic and router overhead. When summarization is enabled, only the summarized routes are propagated into the backbone (Area 0). This provides several critical benefits:
- Reduced LSA Flooding: It prevents topology changes (like a link failure) in one area from being propagated into the backbone and other areas.
- Smaller Routing Tables: It minimizes the number of routing table entries that routers in other areas must maintain.
- Improved Efficiency: Because individual changes are hidden, routers in the backbone and other areas do not have to rerun the SPF algorithm for every change in a remote area, which saves CPU and memory resources.
While ABRs specifically inject these as Type-3 summary LSAs into the backbone, ASBRs summarize Type-5 external LSAs.
Related exam: Modules 8 – 13: Checkpoint Exam: Network Routing Answers (CCNP ENCOR v9)
