When implementing high-availability stateful switchover BGP routing, in which situation would Cisco NSR be required?

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When implementing high-availability stateful switchover BGP routing, in which situation would Cisco NSR be required?

A. On the PE routers connecting to the CE routers which are not NSF aware or are not NSF capable
B. On the PE routers connecting to the CE routers which support graceful restart
C. On the PE routers connecting to the CE routers which are incapable of performing stateful switchover operations because the CE routers are only NSF aware but not NSF capable
D. On the PE routers connecting to the CE routers which are incapable of performing stateful switchover operations because the CE routers are only NSF capable but not NSF aware
E. On the service provider core P routers which are also NSF aware
F. On the service provider core P routers which are also NSF capable