Operation of the CMS Level-1 Calorimeter Trigger in High Pileup Conditions and Motivations for Phase-2

To maintain high trigger efficiencies and stable rates during significant changes to beam conditions throughout 2017, the CMS Level-1 calorimeter trigger required dynamic and flexible operation. Successfully running since 2015, utilising Xilinx Virtex 7 690 FPGAs and 10 Gbps optical links, the versatile design has enabled quick adaption to improve algorithms to mitigate large rates from high pileup and changes in detector response, and as the LHC responded to a number of unexpected challenges. Operational experience and lessons learned will be discussed, and how they will inform important decisions in the design and implementation of the Phase-2 trigger upgrade.