• For network-based URLs (such as TFTP and FTP), you must create an empty file at the configured URL before the switch can write bindings to the binding file at that URL. See the documentation for your TFTP server to determine whether you must first create an empty file on the server; some TFTP servers cannot be configured this way. • To ensure that the lease time in the database is accurate, we recommend that you enable and configure Network Time Protocol (NTP). • If NTP is configured, the switch writes binding changes to the binding file only when the switch system clock is synchronized with NTP.
• Before configuring the DHCP relay agent on your switch, make sure to configure the device that is acting as the DHCP server. You must specify the IP addresses that the DHCP server can assign or exclude, configure DHCP options for devices, or set up the DHCP database agent. • If you want the switch to relay DHCP packets, the IP address of the DHCP server must be configured on the switch virtual interface (SVI) of the DHCP client. • If a switch port is connected to a DHCP server, configure a port as trusted by entering the ip dhcp snooping trust interface configuration command.
• If a switch port is connected to a DHCP client, configure a port as untrusted by entering the no ip dhcp snooping trust interface configuration command. Restrictions for Configuring DHCP We recommend that you do not use transmit (TX) Remote or Encapsulated Remote Switched Port Analyzer (RSPAN or ERSPAN) on VLAN ports which support DHCP Snooping or DHCP Relay Agent. If TX RSPAN or ERSPAN is required, avoid using VLAN ports that are in the forwarding path for DHCP packets. Information About DHCP DHCP Server The DHCP server assigns IP addresses from specified address pools on a switch or router to DHCP clients and manages them.
If the DHCP server cannot give the DHCP client the requested configuration parameters from its database, it forwards the request to one or more secondary DHCP servers defined by the network administrator. The switch can act as a DHCP server. If the DHCP server provides the client with the requested configuration, it will not forward the message to the other server. DHCP Relay Agent A DHCP relay agent is a Layer 3 device that forwards DHCP packets between clients and servers. Relay agents forward requests and replies between clients and servers when they are not on the same physical subnet.
Relay agent forwarding is different from the normal Layer 2 forwarding, in which IP datagrams are switched transparently between networks. Relay agents receive DHCP messages and generate new DHCP messages to send on output interfaces. Configuring DHCP Restrictions for Configuring DHCP Configuring DHCP 2