OSPF Fast Convergence

OSPF fast convergence is an extended feature of OSPF to speed up route convergence. The characteristics of OSPF fast convergence are as follows:

• Priority-based OSPF Convergence
• When certain routes on the network change, only the changed routes are recalculated. This is called Partial Route Calculation (PRC).

• An intelligent timer is used to implement LSA management (the generation and receipt of LSAs). With the intelligent timer, OSPF can respond to infrequent changes quickly, whereas suppress impacts caused by frequent changes.

  To avoid excessive consumption of device resources due to network connections or frequent route flapping, RFC 2328 defines the following rules:

  • After an LSA is generated, it cannot be generated again within 1 second. An LSA is updated at an interval of 5 seconds.
  • The interval for receiving LSAs is 1 second.

  On a stable network where routes need to fast converge, you can use intelligent timers to set the intervals for updating and receiving LSAs to 0 seconds. In this way, topology or route changes can be advertised to the network or be immediately detected, thereby speeding up route convergence on the network.

• An intelligent timer is used control route calculations.

  When the network topology changes, OSPF devices need to recalculate routes. This means that frequent changes in the network topology will affect the performance of devices. To address issue, RFC 2328 requires that a delay timer be used for route calculations. That is, route calculations are performed only after the delay timer expires. However, the delay suggested by RFC 2328 has a fixed value, and cannot ensure both fast response to topology changes and effective suppression of flapping.

  With an intelligent timer, the delay in route calculation can be flexibly set as desired. In this way, OSPF can quickly respond to infrequent changes, whereas suppress impacts caused by frequent changes.

• OSPF Smart-discover