ABOUT OSPF

HI team

 

Its a silly question but i wana know why OSPF has concept that all NON area 0 must go through Area 0 why has Area 0 medatory how it impact............

Comments

  • Praveen,

    Area 0 (aka backbone area) is the area that connects all the other areas and always contain the routing information for all the areas, so it is sort of a "glue" that ties all the other OSPF areas together, so you can have reachability from area 1 to area 2, for example. Therefore, to go from one area to another, you must go through area 0 (there are ways to tweak this behavior with the transit capability under OSPF), but by default you MUST traverse area 0. With all this said, area 0 has to be a normal area, it cannot be a stub by any way, shape or form, as it needs to have all the information for all the areas.

    OSPF is well know to present a DV behavior between areas, as the routers inside a given area only have visibility (LSAs) into the area, and not all LSAs. Every time it needs to go to an IA destination, it should get there by going to its ABR. Area 0 is designed to prevent loops as well.

    HTH

    Good luck!

  • OSPF is well know to present a DV behavior within an area, as the routers inside a given area only have visibility (LSAs) into the area, every time it needs to go to an IA destination, it should get there by going to its ABR. Area 0 is designed to prevent loops as well.

    qqabdal, if you mean OSPF behaves like DV within same area, this is not correct, OSPF uses link state algorithms within an area but it behaves almost like Distance Vector protocol between Areas.

  • Additionally if you have just one ospf area in your network then this area could be either zero or non zero area i.e in this case its not mandatory to have area zero. Though this kind of set up with single area is not recommended in a large routing domain.

  • Its a silly question but i wana know why OSPF has concept that all NON area 0 must go through Area 0 why has Area 0 medatory how it impact............

    Hi Praveen,

    Area 0 is not mandatory if you don't need inter-area route(LSA3)

    Suppose you have two areas R1-Area 1- R2 - Area 2 R3, you can configure OSPF, R2 gets route of R1, R2 gets route of R2 but area 1 doesn't get routes of area 2 and vice-versa.

    But if you need inter-area routes, which is called LSA 3, you should have area 0 because each area has capabilities to generate and advertise LSA 1 and LSA 2 but can't generate LSA3. Only Area 0 has capability to generate LSA 3 (inter-area routes) so every area should connect with Area 0 either directly or using virtual-link.

    Good Luck

  • Narayan - Good catch, I mistyped it and it got transposed. I really meant to say LS behavior inside an area and DV between areas.

    Thanks for pointing that out, and sorry for any confusion that may have caused.

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