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DHCP Snooping
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Chapter 16. DHCP Snooping
16.1 Introduction to DHCP Snooping
For the sake of security, the IP addresses used by online DHCP clients need to be tracked
for the administrator to verify the corresponding relationship between the IP addresses the
DHCP clients obtained from DHCP servers and the MAC addresses of the DHCP clients.
Switches can track DHCP client IP addresses through the DHCP snooping function, which
listens DHCP broadcast packets.
DHCP snooping listens the following two types of packets to retrieve the IP addresses the
DHCP clients obtain from DHCP servers and the MAC addresses of the DHCP clients:
DHCP-ACK packet
DHCP-REQUEST packet
When an unauthorized DHCP server exists in the network, a DHCP client may obtains an
illegal IP address. To ensure that the DHCP clients obtain IP addresses from valid DHCP
servers, you can specify a port to be a trusted port or an untrusted port by the DHCP
snooping function.
Trusted ports can be used to connect DHCP servers or ports of other switches. Untrusted
ports can be used to connect DHCP clients or networks.
Untrusted ports drop the DHCP-ACK and DHCP-OFFER packets received from DHCP
servers. Trusted ports forward any received DHCP packets to ensure that DHCP clients can
obtain IP addresses from valid DHCP servers.
16.2 DHCP Snooping Configuration
Perform following commands in global configuration mode.
Table 16-1 Configure the DHCP snooping function
Operation
Command
Description
Enable DHCP-Snooping
dhcp-snooping
By default,
DHCP-Snooping is
disabled.
Enter interface configuration mode
interface Ethernet port_id
(device/slot/port)
Configure port connected to DHCP
server direction to be Trust
dhcp-snooping trust
By default, all ports are
untrust port.