H3C Technologies H3C S12500 Series Switches - Manual (Page 89)

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Application environment of trusted ports
Configuring a trusted port connected to a DHCP server
Figure 35 Trusted port configuration
As shown in Figure 35 , the trusted port forwards reply messages from the DHCP server to the client, but
the untrusted port connected to the unauthorized DHCP server cannot forward any reply messages. This
makes sure 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 not directly connected to DHCP
clients, from recording clients' IP-to-MAC bindings upon receiving DHCP requests.
Figure 36 Configuring trusted ports in a cascaded network
Trusted port disabled from recording binding entries
DHCP snooping
Switch A
DHCP snooping
Switch C
DHCP client
Host D
DHCP client
Host C
DHCP client
Host B
DHCP server
Device
DHCP snooping
Switch B
GE3/0/4 GE3/0/2
GE3/0/3
GE3/0/1 GE3/0/2
GE3/0/3
GE3/0/4 GE3/0/2 GE3/0/1
GE3/0/3
GE3/0/1
DHCP client
Host A
GE3/0/1
Untrusted port
Trusted port enabled to record binding entries