Reference Guide

Table Of Contents
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
DHCP Snooping
DHCP snooping protects networks from spoofing. In the context of DHCP snooping, ports are either trusted or not
trusted.
By default, all ports are not trusted. Trusted ports are ports through which attackers cannot connect. Manually
configure ports connected to legitimate servers and relay agents as trusted.
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.
FTOS Behavior: Introduced in FTOS version 7.8.1.0, DHCP snooping was available for Layer 3 only and dependent on
DHCP relay agent (ip helper-address). FTOS version 8.2.1.0 extends DHCP snooping to Layer 2 and you do not
have to enable relay agent to snoop on Layer 2 interfaces.
FTOS Behavior: Binding table entries are deleted when a lease expires or when the relay agent encounters a
DHCPRELEASE. Starting with FTOS version 8.2.1.2, 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 made. However, DHCPRELEASE and
DHCPDECLINE packets are allowed so that the DHCP snooping table can decrease in size. After the table usage falls
below the maximum limit of 4000 entries, new IP address assignments are allowed.
NOTE: DHCP server packets are dropped on all not trusted interfaces of a system configured for DHCP snooping.
To prevent these packets from being dropped, configure ip dhcp snooping trust on the server-connected
port.
Enabling DCHP Snooping
To enable DCHP snooping, use the following commands.
1. Enable DHCP snooping globally.
CONFIGURATION mode
ip dhcp snooping
2. Specify ports connected to DHCP servers as trusted.
INTERFACE mode
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