QTech QSW-2800 series, Configuration Manual

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193
DHCP Server DHCP Client
DHCPDiscover(Broadcast)
DHCPOFFER(Unicast)
DHCPREQUEST(Broadcast)
DHCPACK(Unicast)
DHCPDiscover
DHCP Relay
DHCPOFFER
DHCPREQUEST
DHCPACK
DHCP relay
As shown in the above figure, the DHCP client and the DHCP server are in different networks,
the DHCP client performs the four DHCP steps as usual yet DHCP relay is added to the
process.
The client broadcasts a DHCPDISCOVER packet, and DHCP relay inserts its own IP address
to the relay agent field in the DHCPDISCOVER packet on receiving the packet, and forwards
the packet to the specified DHCP server (for DHCP frame format, please refer to RFC2131).
On the receiving the DHCPDISCOVER packets forwarded by DHCP relay, the DHCP server
sends the DHCPOFFER packet via DHCP relay to the DHCP client.
DHCP client chooses a DHCP server and broadcasts a DHCPREQUEST packet, DHCP relay
forwards the packet to the DHCP server after processing.
On receiving DHCPREQUEST, the DHCP server responds with a DHCPACK packet via DHCP
relay to the DHCP client.
DHCP Relay Configuration Task List:
1. Enable DHCP relay.
2. Configure DHCP relay to forward DHCP broadcast packet.
3. Configure share-vlan
1. Enable DHCP relay.
Command Explanation
Global Mode
service dhcp
no service dhcp
DHCP server and DHCP relay is enabled as the
DHCP service is enabled.
2. Configure DHCP relay to forward DHCP broadcast packet.
Command Explanation
Global Mode
ip forward-protocol udp bootps
no ip forward-protocol udp bootps
The UDP port 67 is used for DHCP broadcast
packet forwarding.
Interface Configuration Mode
ip helper-address <ipaddress>
no ip helper-address <ipaddress>
Set the destination IP address for DHCP relay
forwarding;
the “
no ip helper-address