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no ip helper-address A.B.C.D
//This command is used to disable the DHCP relay.
Parameter
None
Default
Disable
Command Mode
Interface configuration mode
Example
//This command is used to open the DHCP relay in vlan 1
switch(config)#interface vlanif1
switch(config-vlanif1)# ip helper-address 192.168.1.1
4.9
DHCP snooping
DHCP snooping configuration commands include:
ip dhcp-snooping
ip dhcp-snooping trust
show ip dhcp-snooping lease
Function Brief
DHCP snooping is a security feature of DHCP, and provides the
following functions: Ensure that a client obtains its IP address from an
authorized server. If an unauthorized DHCP server that is built privately
exists on the network, the DHCP clients may obtain incorrect IP addresses
and network configuration parameters, and consequently cannot implement
communication normally. To ensure that DHCP clients can obtain IP
addresses from an authorized DHCP server, the DHCP snooping security
mechanism supports configuration of ports as trusted or untrusted ports.
1、A trusted port can forward received DHCP packets normally.
2、 On receiving the DHCP-ACK and DHCP-OFFER packets from the
DHCP server, an untrusted port drops the packets.
4.9.1
ip dhcp-snooping
Command Description
ip dhcp-snooping:
//This command is used to enable the DHCP snooping configuration mode.
no ip dhcp-snooping:
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