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Latest post 02-15-2017 7:48 AM by 2 replies.
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  • 02-09-2017 6:15 AM

    IGP convergence and microloops


    Lately i was trying to understand better the effect of having a microloop issue during a reconvergence in EIGRP as opposed to Ospf.
    Understanding deeply the differences between the two situation seems to be not so easy because discussions are more busy to understand how microloop is formed rather than understand how microloop impacts each different protocol.
    Taking the discussion on a higher level perspective the impact of microloops on Eigrp and Ospf seems to me pretty similar so far.
    Once you tweak Ospf timers appropriately (lsa throttling and spf computation) although Ospf could be very slow to reconverge (assuming a large area size and assuming we are not using high speed reconvergence mechanisms) it looks quite similar to how Eigrp reconvergence time happens to be.
    From a theory perspective we know the benefits of using termination detection for diffusing computations which brings awareness on how topology looks like at any moment of reconvergence process but since LSA acknowledgement exists in Ospf too, why only Eigrp seems to be immune to microloops ?

    If the AS is large enough to introduce certain amount of delay regarding update propagation, how Eigrp could have consistent view of the topology at any time of reconvergence thus avoiding microloops ?

    i appreciate any valuable reply to this, thanks

    • Post Points: 20
  • 02-12-2017 7:54 PM In reply to

    • JoeM
    • Top 10 Contributor
    • Joined on 04-15-2012
    • Guadalajara, Mexico
    • Elite
    • Points 31,545

    Re: IGP convergence and microloops

    ....why only Eigrp seems to be immune to microloops?....

    Hi PGallo,  Good to hear from you.

    I don't think EIGRP is completely immune, but it is engineered to be faster than other routing protocols.     What does EIGRP do that the other protocols don't do (out-of-the-box with no modifications)? It attempts to recognize a Feasible Successor for each prefix, in case it is needed, without recalculating everything.

    RUSS WHITE wrote a blog about this a few years ago.  The last paragraph of his blog hints at a couple of the solutions  that go further than even EIGRP does. (fast-reroute and Ordered-FIB)      EIGRP still has to look up and install the Feasible Successor, right?  But with Fast-Reroute, the repair route is already installed and ready to go -- giving it a headstart on normal routing protocol methods.


    Microloop! - blog by Russ White


    Four Steps to Fast Convergence -- Russ White

    1. Discover Failure - How long?

    2. Report - How long to spread the news?

    3. Calculate - How long for new path?  <-- Eigrp tries to do this beforehand.

    4. Install - how long for change to new path?    <-- EIGRP still must install.

    • Post Points: 20
  • 02-15-2017 7:48 AM In reply to

    Re: IGP convergence and microloops

    Hi Joe,


    thank you for the response!

    I was looking to Russ books too honestly. Indeed Eigrp behaves different than Ospf at point 3, imagining a situation where no other reconvergence mechanisms are implemented. Eigrp tends to calculate an alternative path and also runs checks on it.

    Ospf doesn't spread the load of SPF computation across several routers and performs Spf from the perspective of single router /single path (shortest path) because it can do this since it has the view of entire area. Eigrp distribute the load of calculation across several devices because of its partial view of the topology.

    What is not yet clear though is if the main difference is because of feasible successor calculation absence in Ospf that makes it more subject to microloops problems or there is something else underneath we are not considering.

    This means specifically, does Eigrp is not immune to microloops but it tends to resolve a microloop faster than Ospf ? Also it seems that Eigrp drop packets rather than loop them. It would be interesting to investigate possible scenarios to simulate this behavior with examples.



    • Post Points: 5
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