300-420 Question #252: Real Exam Question with Answer & Explanation

Question

A network engineer must optimize a campus OSPF deployment. Currently, each time a type 1 or type 2 LSA is generated within an area, the OSPF process must recompute the entire SPT. Which solution improves the recomputation process?

Options

• ASPF
• BPRC
• CBFD
• DiSPF

Explanation

Incremental SPF (iSPF) optimizes OSPF convergence by recalculating only the subtree of the SPT affected by a topology change rather than rerunning the full Dijkstra algorithm.

Common mistakes.

• A. SPF is the standard full Dijkstra shortest-path computation that recalculates the entire SPT from the root - this is precisely the inefficient behavior the question is asking how to improve.
• B. PRC (Partial Route Calculation) is an optimization for Type 3, 4, 5, and 7 LSA changes (inter-area and external routes), not for Type 1 or Type 2 intra-area LSAs.
• C. BFD (Bidirectional Forwarding Detection) accelerates failure detection by sending sub-second hello packets, but it plays no role in optimizing the SPT recomputation algorithm itself.

Concept tested. OSPF incremental SPF (iSPF) recomputation optimization

Reference. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/xe-3s/iro-xe-3se-book/iro-incremental-spf.html

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