I have heard a lot of people saying ospf uses a backbone area mechanism to avoid routing loops, yet I always failed to understand where and when ospf produces loops in its topology? if anyone can explain this with real lab scenario.. please explain..

I have more to discuss on this.

  • I'll try and get a lab setup tomorrow if no one beats me to it. – Jordan Head Dec 12 '14 at 23:56
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 11 '17 at 0:24

Imagine a network with three areas: 0, 1 and 2. Area 0 is connected to 1 and 2; area 1 is connected to 0 and 2; and area 2 is connected to 0 and 1. Further, imagine that in this network, there is nothing special about area 0 -- in this network, backbone rules don't apply.

Area 1 learns about routes from area 2 from two sources: area 2 and area 0. Now let's say that because of metrics, the shortest path to area 2 is through area 0 (instead of your direct connection). Area 0 also learns about area 2 from area 1 and area 2. But because of misconfigured metrics, the shortest path to area 2 is through area 1.

So if you send data from area 1 to area 2, the packets get forwarded to area 0, but area 0 forwards them back to area 1. This is a classic routing loop.

Instead, if we make area 0 a backbone, and say areas can only receive routes from the backbone, then there is only one possible path to area 2 -- through the backbone. Area 1 will ignore routes being advertised directly from area 2. In this way there are no multiple paths, and therefore no loops.

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    If you find it helpful, you can always mark it accepted ;) – Ron Trunk Dec 13 '14 at 0:07
  • I'm sorry Ravindra, I'm having trouble understanding your question. – Ron Trunk Dec 13 '14 at 0:19
  • Ron, Thank you for the descriptive explanation. First of all, I just want to ask if we dont have backbone area, according to your topology above; would it possible for these area to communicate with each other without having a backbone area?? The answer is "NO" (not mentioning redistribution here) and now when we know two areas can not communicate without having area 0 in the topology, so where the concept of loop exist in ospf?? Well according to me this is just a hypothetical question, and doesn't have its existence. There is surely a concept of Route Feedback but not Routing loop. Thanks – Ravindra Dec 13 '14 at 0:26

I would say traffic passes through backbone area because developer of ospf designed it such a way, they new if they dont bring area 0 it will die. It is more like someone asking why 1+1=2?

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    In my example, I assumed that areas can talk to each other, and that area 0 was just another area. Again this was just to illustrate how loops are created and how having a backbone solves that problem. The purpose of the backbone is to eliminate redundant paths between areas. If all paths must go through the backbone, then there can't be redundant paths. Other routing protocols use split horizon to prevent this problem. – Ron Trunk Dec 13 '14 at 3:00

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