Refer to the Release Notes for Cisco Emergency Responder 8.0 for supported Cisco MCS Unified CM Appliance platforms and their Cisco ER scalability.
Cisco ER performance can be affected if Cisco ER is managing switches across a WAN link. Cisco ER must send SNMP requests to the managed switches, and WAN delays can lead to SNMP timeouts and increase the time needed to track phone and switch changes. You might need to tune the SNMP parameters. See the "Configuring the SNMP Connection" for more information.