Users Guide

• 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.
DHCP snooping is supported on Layer 2 and Layer 3 traffic. DHCP snooping on Layer 2 interfaces does
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 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 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.
Dynamic Host Configuration Protocol (DHCP) 320