Exam 300-420 topic 1 question 216 discussion

C. BFD

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/15-sy/iro-15-sy-book/iro-incre-spf.pdf OSPF uses Dijkstra’s SPF algorithm to compute the shortest path tree (SPT). During the computation of the SPT, the shortest path to each node is discovered. The topology tree is used to populate the routing table with routes to IP networks. When changes to a Type-1 or Type-2 link-state advertisement (LSA) occur in an area, the entire SPT is recomputed. In many cases, the entire SPT need not be recomputed because most of the tree remains unchanged. Incremental SPF allows the system to recompute only the affected part of the tree.

ISPF (Incremental SPF): ISPF, on the other hand, is an optimization technique that minimizes the computational overhead of the SPF algorithm by calculating only the changes in the SPF tree caused by a topology change. It uses the existing shortest path tree and incrementally updates it based on the changes in the network. Instead of recomputing the entire SPF tree, ISPF calculates the shortest paths only for the areas or portions of the network affected by the topology change. It uses the existing SPF tree as a starting point and incrementally updates the routes that are affected by the change. This reduces the computational complexity and convergence time in large networks.

Partial SPF calculation eliminates the need for recalculating the full topology information https://www.networkers-online.com/p/fast-convergence-partial-spf-calculation

OSPF supports a method only to recalculate the part of the SPT that has changed, called incremental SPF. Since you don’t run a full SPF all the time, the CPU load of the router decreases and convergence times improve. On the other hand, your router stores the previous copy of the SPT which requires some extra memory. There are three scenarios where incremental SPF has a positive impact: Adding (or removing) a leaf node to a branch Link failure in non-SPT Link failure in branch of SPT

D. iSPF iSPF (Incremental Shortest Path First) is a feature that optimizes the OSPF route computation process. When iSPF is enabled, OSPF only recomputes the parts of the Shortest Path Tree (SPT) that are affected by a change in the topology, instead of recomputing the entire SPT each time a type 1 or type 2 LSA is generated within an area. This results in reduced CPU usage and faster convergence times.