Ip Address Lease Extension; Dhcp Message Format - H3C S7500E Series Operation Manual

Operation Manual – DHCP
H3C S7500E Series Ethernet Switches
3) If several DHCP servers send offers to the client, the client accepts the first
received offer, and broadcasts it in a DHCP-REQUEST message to formally
request the IP address.
4) All DHCP servers receive the DHCP-REQUEST message, but only the server to
which the client sent a formal request for the offered IP address returns a
DHCP-ACK message to the client, confirming that the IP address has been
allocated to the client, or returns a DHCP-NAK unicast message, denying the IP
address allocation.
Note:
After the client receives the DHCP-ACK message, it will probe whether the IP
address assigned by the server is in use by broadcasting a gratuitous ARP packet.
If the client receives no response within specified time, the client can use this IP
address. Otherwise, the client sends a DHCP-DECLINE message to the server to
request an IP address again.
If there are multiple DHCP servers, IP addresses offered by other DHCP servers
are assignable to other clients.

1.2.3 IP Address Lease Extension

The IP address dynamically allocated by a DHCP server to a client has a lease. After
the lease duration elapses, the IP address will be reclaimed by the DHCP server. If the
client wants to use the IP address again, it has to extend the lease duration.
After the half lease duration elapses, the DHCP client will send the DHCP server a
DHCP-REQUEST unicast message to extend the lease duration. Upon availability of
the IP address, the DHCP server returns a DHCP-ACK unicast confirming that the
client's lease duration has been extended, or a DHCP-NAK unicast denying the
request.
If the client receives the DHCP-NAK message, it will broadcast another
DHCP-REQUEST message for lease extension after 7/8 lease duration elapses. The
DHCP server will handle the request as above mentioned.

1.3 DHCP Message Format

Figure 1-3
format and involves eight types. These types of messages have the same format
except that some fields have different values. The numbers in parentheses indicate the
size of each field in bytes.
gives the DHCP message format, which is based on the BOOTP message
1-3
Chapter 1 DHCP Overview