A DHCP client can send a release message to the server to indicate that the address assigned in the lease is not used. The BNG configured as DHCP relay or relay proxy can also send a release message to the server on behalf of the client in the following cases:
the drop of a DHCPv4 ACK or DHCPv6 reply message caused by a host creation failure, for example, when there are not enough resources or there is a duplicate host
sla-profile or sub-profile host and session limit enforcement
lease time expiration when lease split is active
Subscriber Host Connectivity Verification (SHCV) connectivity loss with action lease removal
an IPoE session timeout
manual clearing of an IPoE session or lease state; a DHCP release message is not sent when the optional no-dhcp-release parameter is specified in the clear service id dhcp lease-state or clear service id dhcp6 lease-state commands.
The BNG does not send a DHCP release message on behalf of the client when the remaining lease time is less than 5 minutes.
The release-include-gi-address command configured in the dhcp context of an interface sets the gateway IP Address (giaddr) field in the DHCPv4 release messages to the configured GI address. By default, the giaddr field in a DHCPv4 release message is transparently forwarded as received from the client or set to 0.0.0.0 when the DHCPv4 release message is sent by the BNG on behalf of the client.