b09f51b5_HCP_Overview__Support____Cisco__DHCP_Option_Name
[DHCP Option Code] 51 [Description] Allows the client to request a lease for the IP address. [DHCP Option Name] 
IP address lease time option
IP Addressing: DHCP Configuration Guide, Cisco IOS XE Release 3S (Cisco ASR 1000) - DHCP Overview [Support] - Cisco