Setup Guide

● 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.
To insert Option 82 into DHCP packets, follow this step.
● Insert Option 82 into DHCP packets.
CONFIGURATION mode
ip dhcp relay information-option [trust-downstream]
For routers between the relay agent and the DHCP server, enter the trust-downstream option.
● Manually reset the remote ID for Option 82.
CONFIGURATION mode
ip dhcp relay information-option remote-id
DHCPv6 relay agent options
The DHCPv6 relay agent inserts Options 18 and 37 before forwarding DHCPv6 packets to the server. These DHCPv6 options
are enabled by default and are not configurable.
Interface ID
(Option 18)
Interface on which the client-originated message is received.
The interface-ID is 12 bytes long and is constructed using three ifindexes: Logical, Received, and Physical.
Each of the ifindex is 4 bytes long.
Remote ID
(Option 37)
Identifies the host from which the message is received.
The default values of the Options 18 and 37 are as follows:
● Default Agent Interface ID is constructed in the format VLANID:LagID:SlotID:PortStr. When the port is fanned-out,
the PortStr is represented as mainPort:subPort (all in ASCII format).
● Default Agent Remote ID is the system MAC address of the relay agent that adds Option 37 (in binary format).
DHCP Snooping
DHCP snooping is a feature that protects networks from spoofing. It acts as a firewall between the DHCP server and DHCP
clients.
DHCP snooping places the ports either in trusted or non-trusted mode. By default, all ports are set to the non-trusted mode.
An attacker can not connect to the DHCP server through trusted ports. While configuring DHCP snooping, manually configure
ports connected to legitimate servers and relay agents as trusted ports.
When you enable DHCP snooping, the relay agent builds a binding table — using DHCPACK messages — containing the client
MAC address, IP addresses, IP address lease time, port, VLAN ID, and binding type. Every time the relay agent receives a
DHCPACK on a trusted port, it adds an entry to the table.
The relay agent checks all subsequent DHCP client-originated IP traffic (DHCPRELEASE, DHCPNACK, and DHCPDECLINE)
against the binding table to ensure that the MAC-IP address pair is legitimate and that the packet arrived on the correct
port. Packets that do not pass this check are forwarded to the server for validation. This checkpoint prevents an attacker
from spoofing a client and declining or releasing the real client’s address. Server-originated packets (DHCPOFFER, DHCPACK,
and DHCPNACK) that arrive on a not trusted port are also dropped. This checkpoint prevents an attacker from acting as an
imposter as a DHCP server to facilitate a man-in-the-middle attack.
Binding table entries are deleted when a lease expires, or the relay agent encounters a DHCPRELEASE, DHCPNACK, or
DHCPDECLINE.
DHCP snooping is supported on Layer 2 and Layer 3 traffic. DHCP snooping on Layer 2 interfaces does not require a relay
agent.
Binding table entries are deleted when a lease expires or when the relay agent encounters a DHCPRELEASE. Line cards maintain
a list of snooped VLANs. When the binding table is exhausted, DHCP packets are dropped on snooped VLANs, while these
packets are forwarded across non-snooped VLANs. Because DHCP packets are dropped, no new IP address assignments are
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Dynamic Host Configuration Protocol (DHCP)