Loops in OSPF

[abdul.qadir5001]

How loops created in OSPF and how to avoid it

[MHM Cisco World]

many case found, and good design is the solution.

[balaji.bandi]

Most Loops you see in any routing Protocol - redistribution(different case BGP), so avoid redistribution  or use Route-map or Tag while you doing so.,

[abdul.qadir5001]

Dear @balaji.bandi 

Can you please draw a simple topology to explain the point that how loop create in loop please. Thanks

[balaji.bandi]

Not sure what IGP Protocol you looking for ( have you google it ?)

here is a good explanation that can help :

https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8606-redist.html

[Joseph W. Doherty]

Perhaps the most common way for a (transient) loop to form is during convergence, when all the routers do not have "current" topology.

This often happens when a "bad news" route change has taken place, like a dropped link.

For example consider router A has a link to routers B and C, both of those having a link to router D.  Router A considers the path to router D "better" via router B than using router C.  However, routers' B and D link fails, which routers B and D know first.  If router B bounces traffic back to router A, while router A still considers router B the best path, you would have a transient loop.

[Martin L]

First of all, OSPF Unique Router ID is first thing that protects loop forming among other things. Among Intra Areas, we have same database and filtering is not allowed. I mean you cannot filter LSA1 or type 2  from database but you can filter those from getting into the IP route table (RIB). But, you can filter types 3 and 5 from other areas and between areas.   For Inter-Area routes and areas, OSPF uses distance-vector logic and principle (split horizon rule).  The main OSFP area concept that also helps avoiding loops (unlike distance-vector protocols) is the fact that other areas must be connected to Area 0, aka backbone area, either physically or via Virtual link.  In case of multiple Inter-Areas, OSPF has major rule that is often forgot: if ABR learns a Type 3 LSA inside a non-backbone area, that ABR ignores this type 3 LSA when calculating its own routes.  This prevents loops when you have 2 ABRs connecting the same Area X to Area 0.  Other thing is that we have per-determined order of selection: intra-area routes are preferred over Inter Area-routes regardless of metric/cost.  This behavior cannot be change for good reasons.

Regards, ML
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