Notable Replies

  1. Hi Rene,

    Good Day :slight_smile:
    I want to know about convergence after topology change . My question is “After Topology change how a router know a prefix have to be remove from its database” ??


  2. Hi Laz,

    What is the job of LS age ?? Thx


  3. Hello Mohammad

    According to Cisco:

    After a default of 30 minutes the router that originated the entry resends the LSA, with a higher sequence number, in a link-state update (LSU), to verify that the link is still active.

    If however, an LSA is expected after 30 minutes and it doesn’t arrive, the information of the LSA is retained until the LS age or the LSA maximum age is reached. This by default is 60 minutes. So it’s kind of like a dead timer, where if no LSA is resent after 60 minutes, the specific LSA is removed from consideration by the OSPF routing calculation.

    I hope this has been helpful!


  4. Hi Rene,

    So Incremental SPF works only on LSA-1 & LSA-2 . It doesnt work on other LSAs .Correct me if I am wrong .Thx


  5. Hello Zaman

    This is a very good question! Shows you’re thinking very analytically. So, changes in LSA types 1 and 2 will always trigger a full SPF calculation unless the Incremental SPF is enabled. Changes in LSA types 3, 4, 5 and 7 will trigger what is called a partial SPF which is different than an incremental SPF calculation. A partial SPF calculation only involves the leaves of the SPF tree. Any changes in these LSAs indicate changes ONLY in the leaves. The SPF tree itself remains unchanged with changes to these LSAs. This is the default behaviour of a Cisco OSPF router.

    So to specifically answer your question, yes, incremental SPF will be triggered only with changes to LSA types 1 and 2.

    To be a little more clear as to the difference between Incremental and partial SPF, here’s a more detailed explanation of the difference.

    * Partial SPF involves changes only to the leaves. No links are changed.
    * In Incremental SPF, the SPF tree will be changed to discover an alternative best path after a link failure. The farther the incremental SPF is run from the root of the tree, the more savings in time and resources compared to a full SPF. If the change is close to the root, there is little performance improvement over full SPF.

    I hope this has been helpful!


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