In the context of configuring an Encapsulated Remote SPAN (ERSPAN), an ERSPAN ID is required to identify the ERSPAN session uniquely. The command 'erspan id' followed by a number is used to set this identifier, which allows for the monitoring of traffic across multiple VLANs as specified in the configuration. References: Cisco's official documentation and training materials on Implementing and Operating Cisco Service Provider Network Core Technologies (SPCOR) would provide detailed steps and explanations for configuring ERSPAN sessions.