Configuration manual

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
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.
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 flooding it on the entire VLAN.
The relay agent strips Option 82 from DHCP responses before forwarding them to the client.
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Dynamic Host Configuration Protocol (DHCP)