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Both can work efficiently but IS-IS is somehow better then OSPF. Service providers use ISIS to add simplicity in design as well as in configuration. Moreover, there will be less CPU cycles involved in ISIS working as compared to OSPF.
both it can deliver efficiently preform but each one has different feature link for example IS-IS can provide the ability to design large networks by building a single large Level1 (L1) area without any hierarchies in IS-IS , something that would be difficult with OSPF. in addition IS-IS supports "Single Topology" mode for IPv6 routing. This means that IPv4 and IPv6 network uses the same SPF calculation. This reduces the complexity and resources needed to run both protocols. With OSPF to support IPv4 and IPv6 both OSPFv2 and OSPFv3 have to be configured separately. Although the OSPF routing protocol works well in MPLS VPN and its results were very acceptable but still IS-IS have advantages that make service providers engineers prefer it rather than OPSF
thanks
Emad
