OSPF Incremental ISPF
Incremental SPF is more efficient than the full SPF algorithm, thereby allowing OSPF to converge faster on a new routing topology in reaction to a network event.
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So how does this work?
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OSPF runs the Dijkstra’s SPF (Shortest Path First) algorithm to calculate the SPT (Shortest Path Tree), finding the shortest path to each destination. OSPF routers in the same area all have the same LSAs; their LSDB is the same, so they build the same SPT. Even when there is just a single change in the network topology, (change to an LSA type 1 and/or LSA type 2), each OSPF router will run a full SPF calculation again and builds a new SPT.
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Running a full SPT calculation when a topology change occurs is a good thing since we’ll have an updated SPT with the shortest paths to all destinations. The downside, however, is that we are also calculating paths that haven’t changed since the last SPF.
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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.
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There are three scenarios where incremental SPF has a positive impact:
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Adding (or removing) a leaf node to a branch
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Link failure in non-SPT
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Link failure in branch of SPT
Incremental SPF can be enabled per router; it’s an interesting feature if you have a lot of routers in a single area and your average CPU load is high because of OSPF. Routers enabled with incremental SPF and routers not enabled with incremental SPF can function in the same internetwork.
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How do you enable it?
R1(config)#router ospf 1
R1(config-router)#ispf
R1(config-router)#end
How to verify its is enabled?
R1#sh ip ospf
Routing Process "ospf 1" with ID 10.0.0.1
Start time: 2d09h, Time elapsed: 00:07:52.356
Supports only single TOS(TOS0) routes
Supports opaque LSA
Supports Link-local Signaling (LLS)
Supports area transit capability
Router is not originating router-LSAs with maximum metric
Initial SPF schedule delay 5000 msecs
Minimum hold time between two consecutive SPFs 10000 msecs
Maximum wait time between two consecutive SPFs 10000 msecs
Incremental-SPF enabled <<----