Administrator Guide

do not pass this check are forwarded to the server for validation. This checkpoint prevents an attacker from spoong 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.
Dell Networking OS Behavior: Introduced in the Dell Networking OS version 7.8.1.0, DHCP snooping was available for Layer 3 only
and dependent on DHCP relay agent (
ip helper-address). The Dell Networking OS 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.
Dell Networking OS Behavior: Binding table entries are deleted when a lease expires or when the relay agent encounters a
DHCPRELEASE. The switch maintains 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 congured for DHCP snooping. To
prevent these packets from being dropped, congure ip dhcp snooping trust on the server-connected port.
Enabling DHCP Snooping
To enable DHCP 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
ip dhcp snooping trust
3. Enable DHCP snooping on a VLAN.
CONFIGURATION mode
ip dhcp snooping vlan
Enabling IPv6 DHCP Snooping
To enable IPv6 DHCP snooping, use the following commands.
1. Enable IPv6 DHCP snooping globally.
CONFIGURATION mode
ipv6 dhcp snooping
2. Specify ports connected to IPv6 DHCP servers as trusted.
INTERFACE mode
ipv6 dhcp snooping trust
3. Enable IPv6 DHCP snooping on a VLAN or range of VLANs.
CONFIGURATION mode
ipv6 dhcp snooping vlan vlan-id
Adding a Static Entry in the Binding Table
To add a static entry in the binding table, use the following command.
Dynamic Host Conguration Protocol (DHCP)
285