Operation Manual - DHCP
Quidway S3900 Series Ethernet Switches-Release 1510 Chapter 1 DHCP Overview
Huawei Technologies Proprietary
1-3
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 server again when seven-eighths of the lease time elapses. The DHCP
server performs the same operations as those described in the previous section.
1.3 DHCP Packet Format
DHCP has eight types of packets. They have the same format, but the values of some
fields in the packets are different. The DHCP packet format is based on that of the
BOOTP packets. The following table describes the packet format (the number in the
brackets indicates the field length, in bytes):
option(variable)
file(128)
sname(64)
chaddr(16)
giaddr(4)
siaddr(4)
yiaddr(4)
ciaddr(4)
flags(2)secs(2)
xid(4)
hops(1)hlen(1)htype(1)op(1)
option(variable)
file(128)
sname(64)
chaddr(16)
giaddr(4)
siaddr(4)
yiaddr(4)
ciaddr(4)
flags(2)secs(2)
xid(4)
hops(1)hlen(1)htype(1)op(1)
Figure 1-2 DHCP packet format
The field meanings are illustrated as follows:
z op: Operation types of DHCP packets: 1 for request packets and 2 for response
packets.
z htype, hlen: Hardware address type and length of the DHCP client.
z hops: Number of DHCP relays which a DHCP packet passes. For each DHCP
relay that the DHCP request packet passes, the field value increases by 1.
z xid: Random number that the client selects when it initiates a request. The
number is used to identify an address-requesting process.
z secs: Elapsed time after the DHCP client initiates a DHCP request.
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