In the following topology:

  • every Backbone router is an ABR.
  • Router 2 belongs to three different areas, area 0, area 1, and area 4.

is this a valid topology for OSPF ? it works as expected and it shows no misbehavior. however I can't find a reference to such topology anywhere on the internet.

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  • 2
    Now it's on the Internet. ;-) But seriously, it's fine. – Ron Trunk Feb 25 '19 at 13:10
  • 3
    The topology is fine, but the lines defining the area borders should go through an ABR, not through the links connecting the ABR to the other area's routers. With OSPF, you cannot have different areas at either end of a link. A router becomes an ABR when it has (at least) one interface in Area 0 and (at least) one interface in a non-zero area - hence the area border is within the ABR. This might seem nitpicking, but once you start with BGP, you'll see that things are different over there, in exactly this aspect. – Marc 'netztier' Luethi Feb 25 '19 at 16:46
  • Thank you Marc and Ron for your comments!. I am aware that lines should go through the ABR, however I have drawn the lines quickly with a mouse so I couldn't afford to be that accurate :) – Forrest Feb 26 '19 at 6:05

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