Dell Configuration Guide for the S4048–ON System 9.14.2.5

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 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.

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.

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.