Router R$15$ has been a working part of a network that uses OSPFv$2.$ An engineer then issues the shutdown command in OSPF configuration mode on R$15.$ Which of the following occurs? R$15$ keeps OSPF neighbors open but does not accept new OSPF neighbors. R$15$ keeps all OSPF configuration but ceases all OSPF activities $($routes$,$ LSDB$,$ neighbors$).$ R$15$ empties its IP routing table of all OSPF routes but keeps its LSDB intact. R$15$ empties its LSDB but keeps OSPF neighbor relationships active.