I dont consider myself an expert on this but the main reason for conversion from EIGRP to OSPF is usually vendor interoperability.
EIGRP is a properitary routing protocol where as OSPF is standards based (albeit several manufacturers have added their extensions to OSPF). All vendor implementations adhere to standards set by IEEE when it comes to OSPF.
Thus if you have a multi vendor environment you would go for OSPF and for a 100% cisco implementation EIGRP would be the weapon of choice.
Also note that redistribution between these protocols is possible so unless necessary dont go for a fork lift conversion to OSPF ! :)
Arvind indicated one very good reason for switching to OSPF.
One other reason I have seen is when networks have become too large for EIGRP, resulting in too many SIA events etc. While most of these can be averted by good design, I find that some people prefer to move on to a protocol like OSPF in some circumstances. The benefits of inter-operability also plays a part in such a decision.
- OSPF is a open standard with multivendor support
- OSPF is a 'must' in a MPLS network (with TE)
- OSPF provides a manageable hierarchy with areas
Whereas EIGRP has some other advantages:
- EIGRP can do unequal-cost loadbalancing (OSPF does only equal-cost lb)
- EIGRP has automatic routesummarization and user-defined route summaries (OSPF has only user-defined route summaries)
- EIGRP can use more variables to choose its best paths (OSPF uses only one).
OSPF can get a bit messy with respect to link-flapping (each relevant link up/down triggers a SPF recalculation in the entire area), the use of this protocol with respect to routermodels (CPU/ram etc) and the design of areas should be carefully considered. The EIGRP network is less CPU-hungry in a 'plug-and-play' network as it doesn't flood the entire network with 'unrelevant' linkflappings (being unrelevant because of the nature of the distance-vector part of the protocol only transmitting a next-hop to a path, not all parts of it).
(btw: you can also use IS-IS in a MPLS-TE network)
There are quite possibly other points as well, but this is at least something to start with.. :)
As far as I see it, the only viable technical keypoints would be the three first I mentioned.. EIGRP is in all a very good protocol and the choice is almost always just a matter of what you are most comfortable with...
With proper tuning and design EIGRP can scale to 800+ routers without breaking a sweat and converge as fast as OSPF. Unless you are introducing non-Cisco routers in your backbone, or have a network that will exceed 900 neighbors, I would recommend staying with EIGRP if that is what is already in place.
There is a great article by Russ White on this exact topic in the latest edition of Packet Magazine:
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