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CLI-based Configuration Guide - IP Unicast Routing

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R009

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OSPF Fast Convergence

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 generating and receiving of LSAs). With the intelligent timer, infrequent changes are responded to quickly, whereas frequent changes are suppressed as desired.

    To avoid excessive consumption of device resources by network connections or due to frequent route flapping, RFC 2328 maintains that:

    • After an LSA is generated, it cannot be generated again within 1 second. The interval for updating LSAs is 5 seconds.
    • The interval for receiving LSAs is one second.

    On a stable network where routes need to be fast converged, you can use the intelligent timer to set the interval for receiving LSAs to 0 seconds. This ensures that topology or route changes can be advertised to the network or be immediately sensed, thus speeding up route convergence on the network.

  • Route calculation is controlled through the intelligent timer.

    When the network topology changes, devices need to recalculate routes according to OSPF. This means that frequent changes in the network topology affect the performance of devices. To address issue, RFC 2328 requires the use of a delay timer in route calculation so that route calculation is performed only after the specified delay. However, the delay suggested by RFC is a fixed value and cannot ensure both fast response to topology changes and effective flapping suppression.

    By means of the intelligent timer, the delay in route calculation can be flexibly set as desired. As a result, infrequent changes are responded to quickly, whereas frequent changes are suppressed as desired.

  • OSPF Smart-discover
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Updated: 2019-05-17

Document ID: EDOC1000174069

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