b09f51b5_HCP_Overview__Support____Cisco__DHCP_Option_Name
[DHCP Option Code] 54 [Description] Identifies the IP address of the selected DHCP server. [DHCP Option Name] 
Server identifier option
IP Addressing: DHCP Configuration Guide, Cisco IOS XE Release 3S (Cisco ASR 1000) - DHCP Overview [Support] - Cisco