Reference Guide

DHCP Snooping
A DHCP client can run on a switch simultaneously with the DHCP snooping feature as follows:
● 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.
Configure Secure DHCP
The following feature is available on the platform, except where noted.
DHCP as defined 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 spoofing 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 identifies 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 offering an IP address to a client spoofing the same MAC
address on a different relay agent.
● assign IP addresses according to the relay agent. This prevents generating DHCP offers in response to requests from an
unauthorized relay agent.
274
Dynamic Host Configuration Protocol (DHCP)