OSPF fast convergence is an extended feature of OSPF implemented to speed up the convergence of routes. It includes the following:
I-SPFIncremental SPF (I-SPF) recalculates only the routes of the changed nodes rather than all the nodes when the network topology changes. This speeds up the calculation of routes.
PRCPartial Route Calculation (PRC) calculates only the changed routes when the routes on the network change.
Intelligent timerAn Open Shortest Path First (OSPF) intelligent timer can dynamically adjust its value according to the user’s configuration and the interval at which an event is triggered such as the route calculation interval, which ensures rapid and stable operation of a network.The OSPF intelligent timer applies the exponential backoff technology so that the value of the timer can reach the millisecond level.
I-SPF (Incremental SPF)
In ISO 10589, the Dijkstra algorithm is adopted to calculate routes. When a node changes on the network, this algorithm is used to recalculate all routes. The calculation lasts a long time and consumes too many CPU resources, thus affecting the convergence speed.
I-SPF improves this algorithm. Except for the first time, only changed nodes instead of all nodes are involved in calculation. The SPT generated at last is the same as that generated by the previous algorithm. This decreases the CPU usage and speeds up the network convergence.
PRC (Partial Route Calculation)
Similar to I-SPF, PRC calculates only the changed routes. PRC, however, does not calculate the shortest path. It updates the routes based on the SPT calculated by I-SPF.
In route calculation, a leaf represents a route, and a node represents a router. The SPT change and leaf change cause the change of routing information, but the SPT change is irrelevant to the leaf change. PRC processes routing information based on SPT or leaf information.
- If the SPT changes, PRC processes the routing information of all leaves on a changed node.
- If the SPT does not change, PRC does not process the routing information on any node.
- If the leaf changes, RPC processes the routing information on the leaf only.
- If the leaf does not change, PRC does not process the routing information on any leaf.
For example, if OSPF is enabled on an interface of a node, the SPT calculated by I-SPF remains unchanged. In this case, PRC updates only the routes of this interface, thus consuming less CPU resources.
PRC working with I-SPF further improves the convergence performance of the network. It is an improvement of the original SPF algorithm.
OSPF Intelligent Timer
On an unstable network, routes are frequently calculated, which consumes a great number of CPU resources. In addition, LSPs that describe the unstable topology are generated and transmitted on the unstable network. Frequently processing such LSAs affects the rapid and stable operation of the entire network.
The OSPF intelligent timer controls route calculation, LSA generation, and LSA receiving to speed up route convergence on the entire network.
The OSPF intelligent timer speeds up route convergence in the following modes:
On a network where routes are repeatedly calculated, the OSPF intelligent timer dynamically adjusts the route calculation according to the user’s configuration and the exponential backoff technology. In this manner, the number of route calculation times and the CPU resource consumption are decreased. Routes are calculated after the network topology becomes table.
On an unstable network, if a router generates or receives LSAs due to frequent topology changes, the OSPF intelligent timer can dynamically adjust its value. No LSA is generated or handled within an interval, which prevents invalid LSAs from being generated and advertised on the entire network.
By default, the OSPF intelligent timer is started and uses the default value.
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