Huawei Quidway S3500 Series Operation Manual page 118

Operation Manual - Network Protocol
Quidway S3500 Series Ethernet Switches
use some dynamic IP addresses for a long period too, but some individuals may only
need temporarily assigned IP addresses for a short period of time.
Commensurate with these demands, DHCP servers provide three types of IP address
allocation policy:
Manual allocation, with which fixed IP addresses are assigned to a small amount
of special hosts such as World Wide Web (WWW) servers.
Automatic allocation, with which fixed IP addresses are assigned to some hosts
connected to the network for the first time and these hosts are allowed to use the
addresses for a long period of time.
Dynamic allocation, with which some addresses are "leased" to clients. Upon the
expiration of the leases, the clients need to request again. In fact, the addresses
assigned to most clients are dynamic addresses.
2) IP address allocation order
DHCP server selects an IP address for a client in the following order:
The IP address bound with the MAC address of the client in the address pool of
the DHCP server.
The client's previous IP address, that is, the address requested in the Requested
IP Addr Option carried in the DHCP_Discover message sent by the client.
A new address allocated from the DHCP server's pool of available addresses.
This address is the one found first in the address pool.
If the DHCP server does not find an available address, it looks up the expired
leased IP addresses and then conflicting IP addresses to find a valid one for
assignment. If the attempt fails, the server reports error.
3) Following are address pools that a DHCP server may have:
Global address pool: it has significance within the scope of the switch and can be
created using the dhcp server ip-pool command in system view.
VLAN interface address pool: it has significance only at the VLAN interface where
it is created using the dhcp select interface command in VLAN interface view
after a valid unicast IP address is assigned to the VLAN interface. Its address
range is the network segment connected to the VLAN interface.
II. Interacting between DHCP client and server
In order to obtain a valid dynamic IP address, a DHCP client should exchange different
information with the server in several stages, which are different in the following three
situations:
1) The first login of DHCP client
In this case, the DHCP client undergoes four stages in order to set up a connection with
a DHCP server.
Discover stage where the DHCP client looks for a DHCP server. In this stage, the
client broadcasts a DHCP_Discover message on the network and only DHCP
servers respond to it.
Huawei Technologies Proprietary
5-2
Chapter 5 DHCP Configuration