Dell Force10 S25N-S50N FTOS Configuration Guide FTOS 8.4.2.7 E-Series TeraScal - Page 323

When DHCP Snooping is enabled, 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 an trusted port, it adds an entry to the table. The relay agent then 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 the server for validation. This check-point prevents an attacker from spoofing a client and declining or releasing the real client's address. Server-originated packets (DHCPOFFER, DHCPACK, DHCPNACK) that arrive on an untrusted port are also dropped. This check-point prevents an attacker from impostering 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, DHCPDECLINE. FTOS Behavior: Introduced in FTOS version 7.8.1.0, DHCP Snooping was available for Layer 3 only and dependent on DHCP Relay Agent (ip helper-address). FTOS 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. FTOS Behavior: Binding table entries are deleted when a lease expires or when the relay agent encounters a DHCPRELEASE. Starting with FTOS Release 8.2.1.2, 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. Since 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. Once the table usage falls below the maximum limit of 4000 entries, new IP address assignments are allowed. FTOS Behavior: In 8.2.1 releases, ip dhcp snooping trust was required on the port-channel interface as well as on channel members. In subsequent releases, it is no longer necessary nor permitted to configure port-channel members as trusted; configuring the port-channel interface alone as trusted is q sufficient, and ports must have the default configuration to be a channel members. When upgrading from 8.2.1 releases, the channel-member configurations are applied first, so when the port-channel is configured, its membership configuration is rejected, since the member ports no longer have the default configuration. In this case, you must manually remove ip dhcp snooping trust on the channel members add the ports to the port-channel. Note: DHCP server packets will be dropped on all untrusted 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. Enable DCHP snooping Step 1 2 3 Task Enable DHCP Snooping globally. Specify ports connected to DHCP servers as trusted. Enable DHCP Snooping on a VLAN. Command Syntax ip dhcp snooping ip dhcp snooping trust ip dhcp snooping vlan Command Mode CONFIGURATION INTERFACE CONFIGURATION Dynamic Host Configuration Protocol | 323