Qtech QSW-2900-24T-AC [109/209] Dhcp packet processing modes

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Figure 1-2 Format of DHCP packets

The field meanings are illustrated as follows:

· op: Operation types of DHCP packets: 1 for request packets and 2 for response packets.

· htype, hlen: Hardware address type and length of the DHCP client.

· hops: Number of DHCP relay agents which a DHCP packet passes. For each DHCP relay agent that

the DHCP request packet passes, the field value increases by 1.

· xid: Random number that the client selects when it initiates a request. The number is used to identify

an address-requesting process.

· secs: Elapsed time after the DHCP client initiates a DHCP request.

· flags: The first bit is the broadcast response flag bit. It is used to identify that the DHCP response

packet is sent in the unicast or broadcast mode. Other bits are reserved.

· ciaddr: IP address of a DHCP client.

· yiaddr: IP address that the DHCP server assigns to a client.

· siaddr: IP address of the DHCP server.

· giaddr: IP address of the first DHCP relay agent that the DHCP client passes after it sent the request

packet.

· chaddr: Hardware address of the DHCP client.

· sname: Name of the DHCP server.

· file: Name of the start configuration file that the DHCP server specifies for the DHCP client.

· option: Optional variable-length fields, including packet type, valid lease time, IP address of a DNS

server, and IP address of the WINS server.

6.5 DHCP Packet Processing Modes

After the DHCP server is enabled on a device, the device processes the DHCP packet received from a DHCP

client in one of the following three modes depending on your configuration:

· Global address pool: In response to the DHCP packets received from DHCP clients, the DHCP server

picks IP addresses from its global address pools and assigns them to the DHCP clients.

· Interface address pool: In response to the DHCP packets received from DHCP clients, the DHCP