OSPF Features

3.4 You are concerned about the convergence times and CPU resources consumed in the backbone OPSF area. Configure all routers in the backbone so that they only recalculate a portion of the Shortest Path Tree when they receive local link state advertisements.

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12 Responses to “Cisco R/S Lab Exam Challenge – OSPF Features, Part 1 of 2”

  1. Colin West says:

    router ospf

    This will only run SPF over the affected portion of the tree, thus saving CPU resources.

    Col West

    CCIE# 20532

  2. Colin West says:

    Forgot to add the process-id to the router ospf command!! Doh!!

    router ospf 1

  3. noairbag says:

    I came across this once – I remember having to hunt for it and found it under IS-IS rather than OSPF.

  4. Jeff says:

    For OSPF, I’ve only seen it in the Config Guide. For IS-IS, I’ve seen it in both the Config Guide as well as the command reference documentation.

  5. luisgarcia says:

    Yeah seems its the ispf. DocCD says its for changes to type 1 and 2 LSA’s and saves CPU resources by recomputing only a portion of the tree. But I’ve only seen it on 6500′s with 2 Sup cards.

    Its a stretch, but timers lsa arrival could reduce the SPF calculations.But that would have to be the same LSA ID number.

  6. Issa says:

    use ispf command under router ospf

  7. bam says:

    We really should get in the habit of including the DocCD reference…

  8. amit chopra says:

    router ospf 1

    From DOCCD—

    “The topology tree is used to populate the routing table with routes to IP networks. When changes to a Type-1 or Type-2 link-state advertisement (LSA) occur in an area, the entire SPT is recomputed. In many cases, the entire SPT need not be recomputed because most of the tree remains unchanged. Incremental SPF allows the system to recompute only the affected part of the tree. Recomputing only a portion of the tree rather than the entire tree results in faster OSPF convergence and saves CPU resources. Note that if the change to a Type-1 or Type-2 LSA occurs in the calculating router itself, then the full SPT is performed.

    Incremental SPF is scheduled in the same way as the full SPF. Routers enabled with incremental SPF and routers not enabled with incremental SPF can function in the same internetwork.”

  9. ravi says:

    In most of the cases spf time is not the biggest contributor to the overall IGP convergence. Although, ispf relatively faster (time to compute may vary). We also need to consider possibility of reducing IGP routes, by either area filters, or area summarization.

    Convergence time = spf+route-calculation+download time.

  10. IPv6Freely says:

    This is one of those things that I’m not sure how exactly I was supposed to even know it existed…


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