Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.112.0P1 - Page 291

DHCP Snooping A DHCP client can run on a switch simultaneously with the DHCP snooping feature as follows: • If you enable DHCP snooping globally on a switch and you enable a DHCP client on an interface, the trust port, source MAC address, and snooping table validations are not performed on the interface by DHCP snooping for packets destined to the DHCP client daemon. The following criteria determine packets destined for the DHCP client: • DHCP is enabled on the interface. • The user data protocol (UDP) destination port in the packet is 68. • The chaddr (change address) in the DHCP header of the packet is the same as the interface's MAC address. • An entry in the DHCP snooping table is not added for a DHCP client interface. DHCP Server A switch can operate as a DHCP client and a DHCP server. DHCP client interfaces cannot acquire a dynamic IP address from the DHCP server running on the switch. Acquire a dynamic IP address from another DHCP server. Virtual Router Redundancy Protocol (VRRP) Do not enable the DHCP client on an interface and set the priority to 255 or assign the same DHCP interface IP address to a VRRP virtual group. Doing so guarantees that this router becomes the VRRP group owner. To use the router as the VRRP owner, if you enable a DHCP client on an interface that is added to a VRRP group, assign a priority less than 255 but higher than any other priority assigned in the group. DHCP Client on a Management Interface These conditions apply when you enable a management interface to operate as a DHCP client. • The management default route is added with the gateway as the router IP address received in the DHCP ACK packet. It is required to send and receive traffic to and from other subnets on the external network. The route is added irrespective when the DHCP client and server are in the same or different subnets. The management default route is deleted if the management IP address is released like other DHCP client management routes. • ip route for 0.0.0.0 takes precedence if it is present or added later. • Management routes added by a DHCP client display with Route Source as DHCP in the show ip management route and show ip management-route dynamic command output. • Management routes added by DHCP are automatically reinstalled if you configure a static IP route with the ip route command that replaces a management route added by the DHCP client. If you remove the statically configured IP route using the no ip route command, the management route is reinstalled. Manually delete management routes added by the DHCP client. • To reinstall management routes added by the DHCP client that is removed or replaced by the same statically configured management routes, release the DHCP IP address and renew it on the management interface. • Management routes added by the DHCP client have higher precedence over the same statically configured management route. Static routes are not removed from the running configuration if a dynamically acquired management route added by the DHCP client overwrites a static management route. • Management routes added by the DHCP client are not added to the running configuration. NOTE: Management routes added by the DHCP client include the specific routes to reach a DHCP server in a different subnet and the management route. Dynamic Host Configuration Protocol (DHCP) 291