Updating Ip Address Lease - H3C S3100-52P Operation Manual

Operation Manual – DHCP
H3C S3100-52P Ethernet Switch
3) Select: In this phase, the DHCP client selects an IP address. If more than one
DHCP server sends DHCP-OFFER packets to the DHCP client, the DHCP client
only accepts the DHCP-OFFER packet that first arrives, and then broadcasts a
DHCP-REQUEST packet containing the assigned IP address carried in the
DHCP-OFFER packet.
4) Acknowledge: In this phase, the DHCP servers acknowledge the IP address.
Upon receiving the DHCP-REQUEST packet, only the selected DHCP server
returns a DHCP-ACK packet to the DHCP client to confirm the assignment of the
IP address to the client, or returns a DHCP-NAK packet to refuse the assignment
of the IP address to the client. When the client receives the DHCP-ACK packet, it
broadcasts an ARP packet with the assigned IP address as the destination
address to detect the assigned IP address, and uses the IP address only if it does
not receive any response within a specified period.
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 and
requests 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 Updating IP Address Lease

After a DHCP server dynamically assigns an IP address to a DHCP client, the IP
address keeps valid only within a specified lease time and will be reclaimed by the
DHCP server when the lease expires. If the DHCP client wants to use the IP address for
a longer time, it must update the IP lease.
By default, a DHCP client updates its IP address lease automatically by unicasting a
DHCP-REQUEST packet to the DHCP server when half of the lease time elapses. The
DHCP server responds with a DHCP-ACK packet to notify the DHCP client of a new IP
lease if the server can assign the same IP address to the client. Otherwise, the DHCP
server responds with a DHCP-NAK packet to notify the DHCP client that the IP address
will be reclaimed when the lease time expires.
If the DHCP client fails to update its IP address lease when half of the lease time
elapses, it will update its IP address lease by broadcasting a DHCP-REQUEST packet
to the DHCP servers again when seven-eighths of the lease time elapses. The DHCP
server performs the same operations as those described above.
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Chapter 1 DHCP Overview