OSPF Fast Convergence

OSPF fast convergence is an extended feature of OSPF to speed up the convergence of routes. It includes the following components:

Partial Route Calculation (PRC): calculates only those routes which have changed when the network topology changes.
An OSPF intelligent timer: can dynamically adjust its value based on the user's configuration and the interval at which an event is triggered, such as the route calculation interval, which ensures rapid and stable network operation.

OSPF intelligent timer uses the exponential backoff technology so that the value of the timer can reach the millisecond level.

PRC

When a node in a network topology changes, the Dijkstra algorithm needs to recalculate all routes on the network. This calculation takes a long time and consumes a large number of CPU resources, which affects the convergence speed on the entire network. However, PRC uses only the nodes that have changed to recalculate routes, thereby decreasing CPU usage.

In route calculation, a leaf represents a route, and a node represents a device. 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 calculates all the leaves only on the changed node.
If the SPT remains unchanged, PRC calculates only the changed leaves.

For example, if a new route is imported, the SPT of the entire network remains unchanged. In this case, PRC updates only the interface route for this node, thereby reducing the CPU usage.

OSPF Intelligent Timer

On an unstable network, routes are calculated frequently, which consumes a great number of CPU resources. In addition, link-state advertisements (LSAs) 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.

To speed up route convergence on the entire network, the OSPF intelligent timer controls route calculation, LSA generation, and LSA receiving.

The OSPF intelligent timer works as follows:

On a network where routes are calculated repeatedly, the OSPF intelligent timer dynamically adjusts the route calculation based on user's configuration and the exponential backoff technology. The number of route calculation times and the CPU resource consumption are decreased. Routes are calculated after the network topology stabilizes.
On an unstable network, if a router generates or receives LSAs due to frequent topology changes, the OSPF intelligent timer can dynamically adjust the interval. No LSAs are generated or processed within an interval, which prevents invalid LSAs from being generated and advertised on the entire network.