What Nexus technology avoids blocking links between the access layer and the aggregation layer in a data center network design?

FCoE (Fibre Channel over Ethernet) is a storage networking encapsulation technology and is unrelated to preventing STP-blocked uplinks between switching layers.

LACP (Link Aggregation Control Protocol) negotiates port-channel formation but does not by itself prevent STP blocking between two different upstream switches.

vPC presents two physically separate Nexus switches as a single logical port-channel peer to the access layer device. This eliminates the STP blocking that would normally occur on redundant uplinks in a traditional design, allowing all uplinks between the access and aggregation layers to forward traffic simultaneously and maximizing bandwidth utilization.

FEX (Fabric Extender) is a technology for extending switch ports to remote line cards; it does not address blocked links in the aggregation-to-access path.