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  • 1.  Question on Generate Route

    Posted 07-20-2022 03:01



    A common topology used to explain the use of a generate route is the following:


    R1 ------  R2 ---------------  ISP

        OSPF            BGP


    On R2, we configure a generate default route (0/0) that becomes active only if R2 receives a BGP route of 10/16 from the ISP. This generate default route on R2 will have a next hop of the ISP. R2 then redistributes this default route into OSPF. R1 now uses the default route to forward traffic exiting the local OSPF network.


    All good. Except for the fact that the same default route on R2 will be used to forward traffic towards 11/16, which will then be blackholed at the ISP, which means that the generate default route on R2 can cause blackholing of traffic, which is bad.


    Is there a better scenario that highlights the usefulness of a generate route?





    Juniper Business Use Only

  • 2.  RE: Question on Generate Route
    Best Answer

    Posted 07-20-2022 19:57
    I'd say your scenario is not correct because:
    • an ISP is NOT going to send RFC 1918 addresses to anybody
      • ISP will send at the choice of the customer 
        • full tables
        • partial tables
        • default route
    So for default route distribution from ISP into the OSPF domain we would typically ask for the default route to be included from the ISP and import that into OSPF.  Then the status of the ISP will directly determine the correct default route usage.

    And the ISP will have routes for the public addresses 11/16 because they are not private and would be part of the internet domain.

    I suspect the example you are seeing is from old documentation that is being sloppy in the use of example ip addresses with incorrect usage of private RFC 1918 space and using random public addresses instead of the assigned documentation ranges.

    A better example for the generate route would be on edge internal routers for the domain.  Create the generate route with a forward towards the core routers that will become active whenever the edge areas contributing routes are available. 

    The more detailed routes will be used and the generate route allows aggregation  to the core area and the ability to send traffic for the entire prefix to a core router even when specific routes are not available.  This can prevent blackhole of unknown destinations without having to install static routes.

    Steve Puluka BSEET - Juniper Ambassador
    IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP - Retired)

  • 3.  RE: Question on Generate Route

    Posted 07-25-2022 00:45
    Got it. Thanks Steve!