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I found online that the router ID must be unique within the OSPF domain. But what exactly is an OSPF domain (an area?). Let's take the following topology:

enter image description here

Can R1 and R5 have the same router ID, as they are not the same area? If yes, I guess that R4's router ID has to be unique in both area 1 and 2, correct?

Thank you for your help.

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  • By the way, your drawing would never work because an OSPF area can only communicate with the backbone area (Area 0). Area 1 cannot communicate directly with Area 2.
    – Ron Maupin ♦
    Commented Jun 26, 2019 at 17:30
  • @RonMaupin Thank you. That was just a quick drawing to exemplify my question, not an actual work.
    – Nakrule
    Commented Jun 26, 2019 at 17:39

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The OSPF domain (also called an AS) is the collection of OSPF areas connected to the OSPF backbone.

RFC 2328, OSPF Version 2 defines the term, Router ID:"

    Router ID
        A 32-bit number assigned to each router running the OSPF
        protocol.  This number uniquely identifies the router within
        an Autonomous System.

-and-

Router ID
    A 32-bit number that uniquely identifies this router in the AS.
    One possible implementation strategy would be to use the
    smallest IP interface address belonging to the router. If a
    router's OSPF Router ID is changed, the router's OSPF software
    should be restarted before the new Router ID takes effect.  In
    this case the router should flush its self-originated LSAs from
    the routing domain (see Section 14.1) before restarting, or they
    will persist for up to MaxAge minutes.
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