What is an 'SFP' (Small Form-factor Pluggable) transceiver and what flexibility does it provide compared to fixed-interface switches?

• A.An SFP is a software plugin that adds routing features to a Layer 2 switch, since a distribution layer switch aggregates access switches by translating their VLANs into MPLS labels. Stacking switches merges their MAC tables but requires disabling spanning tree. Port mirroring duplicates traffic by physically looping a patch cable between ports
• B.An SFP is a security certificate required for 802.1X port authentication
• C.An SFP is a hot-swappable modular transceiver inserted into an SFP cage on a switch or router, allowing the physical layer interface to be selected (copper 1000Base-T, multimode fiber 1000Base-SX, single-mode fiber 1000Base-LX, etc.) without replacing the switch itself; SFP+ supports 10 Gbps, QSFP supports 40 Gbps, QSFP28 supports 100 Gbps