Change the domain of MDS_1 to 20 is the correct fix under Cisco SAN troubleshooting practice. The issue described involves VSAN, where the operating state depends on the correct combination of port type, VSAN, fabric service, and access-control database. MDS and Nexus SAN features do not infer missing policy: if the active zoneset, fabric-binding database, fcdomain, NPIV setting, or FCoE/LLDP negotiation is wrong, the link may come up physically but the initiator will not reach the target. The selected answer corrects the missing or mismatched element. The remaining choices either act at the wrong layer, change a non-causal parameter, or provide only a temporary reset. After applying the fix, a proper Cisco validation path is to review show interface, show flogi database, show fcns database, show zoneset active, and any isolation or merge-failure logs. That confirms the storage network is operationally consistent.