Hi Rene,
Hope you are doing good. I learned while studying OSPF, that loops are formed in OSPF. Can you help me understand in which scenario loops are formed, and how they are avoided.
Thank you
There are two different areas of consideration with OSPF for this: Intra-Area and Inter-Area.
Intra-Area loops are almost unheard of given the link-state nature of OSPF. Each router has the same database and each runs the same algorithm to determine shortest paths. Aside from a blatant bug an in OSPF implementation code, there is something called a “micro-loop” which will happen when full convergence hasn’t yet happened. Suppose you have one particular router in an area that has very limited hardware resources and is slow to finish its OSPF convergence. It is possible that because of this, there might be a very temporary loop. Other than bumping up the hardware on the router in question, there isn’t much you can do.
The second consideration is Inter-Area. This is where members in an OSPF area don’t have complete knowledge of another Area. Instead, they depend on the ABR to tell them what they need to know. In this respect, OSPF acts more like a distance-vector routing protocol than a link-state one. For this case, I suggest you read through one of the finest documents written on this topic by the great Petr Lapukhov
http://blog.ine.com/wp-content/uploads/2011/01/Loop-Prevention-in-OSPF.pdf