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• If you enable DHCP snooping globally on a switch and you enable a DHCP client on an interface, the trust port, source MAC address,
and snooping table validations are not performed on the interface by DHCP snooping for packets destined to the DHCP client daemon.
The following criteria determine packets destined for the DHCP client:
• DHCP is enabled on the interface.
• The user data protocol (UDP) destination port in the packet is 68.
• The chaddr (change address) in the DHCP header of the packet is the same as the interface’s MAC address.
• An entry in the DHCP snooping table is not added for a DHCP client interface.
DHCP Server
A switch can operate as a DHCP client and a DHCP server. DHCP client interfaces cannot acquire a dynamic IP address from the DHCP
server running on the switch. Acquire a dynamic IP address from another DHCP server.
Virtual Router Redundancy Protocol (VRRP)
Do not enable the DHCP client on an interface and set the priority to 255 or assign the same DHCP interface IP address to a VRRP virtual
group. Doing so guarantees that this router becomes the VRRP group owner.
To use the router as the VRRP owner, if you enable a DHCP client on an interface that is added to a VRRP group, assign a priority less than
255 but higher than any other priority assigned in the group.
Congure Secure DHCP
DHCP as dened by RFC 2131 provides no authentication or security mechanisms. Secure DHCP is a suite of features that protects
networks that use dynamic address allocation from spoong and attacks.
• Option 82
• DHCP Snooping
• Dynamic ARP Inspection
• Source Address Validation
Option 82
RFC 3046 (the relay agent information option, or Option 82) is used for class-based IP address assignment.
The code for the relay agent information option is 82, and is comprised of two sub-options, circuit ID and remote ID.
Circuit ID
This is the interface on which the client-originated message is received.
Remote ID This identies the host from which the message is received. The value of this sub-option is the MAC address of
the relay agent that adds Option 82.
The DHCP relay agent inserts Option 82 before forwarding DHCP packets to the server. The server can use this information to:
• track the number of address requests per relay agent. Restricting the number of addresses available per relay agent can harden a
server against address exhaustion attacks.
• associate client MAC addresses with a relay agent to prevent oering an IP address to a client spoong the same MAC address on a
dierent relay agent.
• assign IP addresses according to the relay agent. This prevents generating DHCP oers in response to requests from an unauthorized
relay agent.
The server echoes the option back to the relay agent in its response, and the relay agent can use the information in the option to forward a
reply out the interface on which the request was received rather than ooding it on the entire VLAN.
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Dynamic Host Conguration Protocol (DHCP)