H3C Technologies H3C S5120 Series Switches - Manual (Page 255)

3-2
Recording IP-to-MAC mappings of DHCP clients
DHCP snooping reads DHCP-REQUEST messages and DHCP-ACK messages from trusted ports to
record DHCP snooping entries, including MAC addresses of clients, IP addresses obtained by the
clients, ports that connect to DHCP clients, and VLANs to which the ports belong.
Application Environment of Trusted Ports
Configuring a trusted port connected to a DHCP server
Figure 3-1 Configure trusted and untrusted ports
Trusted
DHCP server
DHCP snooping
Untrusted Untrusted
Unauthorized
DHCP server
DHCP client
DHCP reply messages
As shown in Figure 3-1 , a DHCP snooping device’s port that is connected to an authorized DHCP
server should be configured as a trusted port to forward reply messages from the DHCP server, so that
the DHCP client can obtain an IP address from the authorized DHCP server.
Configuring trusted ports in a cascaded network
In a cascaded network involving multiple DHCP snooping devices, the ports connected to other DHCP
snooping devices should be configured as trusted ports.
To save system resources, you can disable the trusted ports, which are indirectly connected to DHCP
clients, from recording clients’ IP-to-MAC bindings upon receiving DHCP requests.