Install Guide

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.
NOTE: In DHCP relay agent, configure DHCP snooping such that the packet from DHCP client must not pass through
DHCP snooping-enabled switches twice before reaching the DHCP server.
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 non-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.
DHCP Snooping for a Multi-Tenant Host
You can configure the DHCP snooping feature such that multiple IP addresses are expected for the same MAC address. You
can use the ip dhcp snooping command multiple times to map the same MAC address with different IP addresses. This
configuration is also used for dynamic ARP inspection (DAI) and source address validation (SAV). The DAI and SAV tables
reflect the same entries in the DHCP snooping binding table.
NOTE:
If you enable DHCP Option 82 using the ip dhcp relay command, by default, the remote-ID field contains the
MAC address of the relay agent. If you configure the remote ID as the host name in a VLT setup, configure different host
names on both VLT peers. If you configure the remote ID with your own string, ensure that your strings are different on
both VLT peers.
DHCP Snooping in a VLT Setup
In a VLT setup, the DHCP snooping binding table synchronizes between the VLT nodes. Similarly, the DAI and SAV tables also
synchronize between VLT nodes. For this feature to work in a VLT setup, make sure that DHCP relay, DHCP snooping, SAV, and
DAI configurations are identical between the VLT peer nodes.
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
INTERFACE PORT EXTENDER mode
ip dhcp snooping trust
3. Enable DHCP snooping on a VLAN.
CONFIGURATION mode
ip dhcp snooping vlan name
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Dynamic Host Configuration Protocol (DHCP)