Why doesn't OSPF redistribute secondary address unless "redistribute connected"?

Discussion in 'Cisco' started by worldwidestar, May 17, 2007.

  1. Hello, everyone.

    I make a redistribution experiment, about redistribution between OSPF
    and EIGRP.

    The topology is:

    R2 (E0)--------------------(E0) R3 (S0)---------------------
    (S0) R1

    R2
    E0: 192.168.33.2 / 24

    R3
    E0: 192.168.33.1 / 24
    S0: 192.168.0.6 / 30

    R1
    S0: 192.168.0.5 / 30

    OSPF run between R2 and R3, while EIGRP run between R3 and R1.
    Redistribution is configured on R3.

    Before redistributing, R1 does not know the network 192.168.33.0.
    And then I configure basic redistribution commands, everything goes
    well, network 192.168.33.0 is successfully redistributed into EIGRP
    area, and is known by R1.
    After that, I add a secondary ip address onto R3's E0, and add network
    172.16.0.0 into OSPF process, that is

    R3
    E0: 192.168.33.1 / 24 172.16.0.1 / 16 (secondary)
    S0: 192.168.0.6 / 30

    router ospf 1
    ......
    network 172.16.0.1 0.0.0.0 area 0
    ......

    I thought the network 172.16.0.0 can also be redistributed into EIGRP,
    but it does not appear in R1's routing table.
    Very strange, if I add command "redistribute connected", 172.16.0.0
    can be redistributed into EIGRP, and can be known by R1.

    What's the the reason, please?
     
    worldwidestar, May 17, 2007
    #1
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  2. worldwidestar

    HACabrera Guest


    I've just read you post... It seems I got an answer for you...

    I'm studying for my BSCI, just finished the "Manipulating Routing
    Updates" chapter and this is what I read on my book...


    "EIGRP and OSPF use an interface's primary IP address as the source of
    their updates. They expect the routers on both sides of a link to
    belong to the same subnet"


    As you can see, that is something Protocol-related, not a Cisco's IOS
    programming problem...

    I hope this helps...


    HACabrera
     
    HACabrera, May 24, 2007
    #2
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  3. Split horizon. There is no need to advertise the secondary IP on E0 to other
    routers on E0 because by the rules of IP addressing, ALL routers on E0 must
    have all primary and secondary subnets identically defined.

    Good luck and have fun!
     
    Vincent C Jones, Jun 5, 2007
    #3
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