OSPF Fast Convergence

OSPF fast convergence speeds up the convergence of routes. It includes the following components:

Incremental SPF (I-SPF): recalculates only the routes of the changed nodes rather than all the nodes when the network topology changes, which speeds up route calculation.
Partial Route Calculation (PRC): calculates only the changed routes when the routes on the network change.
OSPF intelligent timer: can dynamically adjust its value based on the user's configuration and the frequency of a triggering event, such as route calculation, which ensures rapid and stable network operation.

The OSPF intelligent timer uses the exponential backoff technology so that the value of the timer is accurate to milliseconds.

I-SPF

In ISO 10589, the Dijkstra algorithm was adopted to calculate routes. When a node changes on the network, this algorithm is used to recalculate all routes, which takes a long time and consumes too many CPU resources, affecting the convergence speed.

I-SPF improves the Dijkstra 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 using the Dijkstra algorithm. I-SPF consumes fewer CPU resources and speeds up network convergence.

PRC

Similar to I-SPF, PRC performs calculation only for the changed routes. The difference is that PRC does not calculate the shortest paths. It updates routes based on the SPTs calculated by I-SPF.

In route calculation, a leaf represents a route, and a node represents a router. Either an SPT change or a leaf change causes a routing information change. The SPT change is irrelevant to the leaf change. PRC processes routing information as follows:

If the SPT changes, PRC processes the routing information of all leaves on a changed node.
If the SPT remains unchanged, PRC does not process the routing information on any node.
If a leaf changes, RPC processes the routing information on the leaf only.
If a leaf remains unchanged, 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. PRC updates only the routes of this interface, consuming fewer CPU resources.

PRC improves the SPF algorithm. Working with I-SPF, RPC further improves network convergence performance.

On live networks, only I-SPF and PRC are used to calculate OSPF routes.

OSPF Intelligent Timer

On an unstable network, routes are calculated frequently, which consumes a great number of CPU resources. In addition, LSAs that describe the unstable topology are generated and transmitted on the unstable network. Frequent processing of data such LSAs affects the rapid and stable operation of the entire network.

To speed up route convergence on the entire network, you can use the OSPF intelligent timer that can control route calculation, LSA generation, and LSA receipt.

The OSPF intelligent timer works in either of the following modes:

On a network where routes are calculated repeatedly, the OSPF intelligent timer dynamically adjusts the interval between two route calculations based on user's configuration and the exponential backoff technology. In this way, the number of route calculations can be reduced, and therefore fewer CPU resources are consumed. Routes will be calculated again after the network topology stabilizes.
On an unstable network, a router generates or receives numerous LSAs due to frequent topology changes. In this case, the OSPF intelligent timer can dynamically adjust the interval at which the LSAs are generated or processed. Within the interval, no LSAs are generated or processed. This prevents invalid LSAs from being generated and advertised on the entire network.

The OSPF intelligent timer is started by default and has a default value.