Reference Guide
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
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 3 interfaces depends on the configured
DHCP relay agent (ip helper-address). 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
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.
INTERFACE mode
ip dhcp snooping trust
3. Enable DHCP snooping on a VLAN.
CONFIGURATION mode
ip dhcp snooping vlan name
Dynamic Host Configuration Protocol (DHCP)
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