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OSPF fast convergence is an extended feature of OSPF implemented to speed up the convergence of routes. It includes the following:
  • I-SPF
    Incremental 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.
  • PRC
    Partial Route Calculation (PRC) calculates only the changed routes when the routes on the network change.
  • Intelligent timer
    An 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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9 Comments

  1. Luiz Carlos

    Eu sei que os roteadores Cisco suportam ISPF (Incremental SPF).
    Minha dúvida é: roteadores Huawei suportam?
    Por favor, respondam para o meu e-mail.

    Obrigado,
    Luiz Carlos.

  2. mammoura

    yes its also supported in Huawei routers

    sim, é também apoiada em roteadores Huawei

  3. Luiz Carlos

    OK. How do I configure Huawei routers with ISPF?
    Example: in Cisco routers the roadmap is:
    #conf t
    #router ospf 10
    #ispf

    And for Huawei?

    Thanks.
    Luiz Carlos.

  4. mammoura

    By Default the ISPF is enabled in huawei and you can set the intelligent timer like below
    >system-view
    #OSPF 10
    #spf-schedule-interval { interval1 | intelligent-timer max-interval start-interval hold-interval [ conservative ] | millisecond interval2 }

  5. Luiz Carlos

    Thanks for the tip before.

    I’m trying the eNSP simulator and I selected “Download Simulator” on your site and you sent me the URL link for this version: eNSP V100R002C00B350 Setup.exe.
    But there is a new version available for download: eNSP V100R002C00B360 Setup.exe.
    There is a catch: this version only speaks Chinese. Is there a new version that speaks English?
    Thanks. Luiz Carlos.

  6. mammoura

    the new version is still in Chinese

  7. Luiz Carlos

    Thanks for the quick answer.
    You have helped me too much.
    What can I do for you?
    Luiz Carlos.

  8. mammoura

    just share the blog with your friends , thats all , so every body can take the benefit

  9. Luiz Carlos

    Done! Now my friends know that “YOU ARE THE GUY”.
    By the way, about ISPF in Huawei that you’ve said that is enbled by default, I need make comparisons between with and without ISPF.
    So, is it possible to disable ISPF in Huawei routers?
    Thanks. Luiz Carlos.

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