Dell EMC SmartFabric OS10 User Guide Release 10.5.0

DHCP relay on VTEPs

Dynamic Host Configuration Protocol (DHCP) clients on hosts in the overlay communicate with a DHCP server using a DHCP relay on the VTEP switch. To work seamlessly, VTEP DHCP relay transmits the virtual-network IP address of the relay interface to the DHCP server.

By default, DHCP uses the giaddr packet field to carry these addresses to the server. In a VxLAN, which has overlay and underlay subnets in the same default VRF, DHCP relay on VTEPs operates without user intervention. However, in a VXLAN in which the underlay and overlay are in different VRFs, the default DHCP method is not successful. The IP tenant subnet is in the overlay address space. The IP address where the VTEP is reachable is in the underlay address space. To transmit the IP subnet of the client separately from the IP address where the VTEP is reachable, you must configure an additional DHCP sub-option (5 or 151) in DHCP relay agent option 82.

Because OS10 does not support the required sub-options in DHCP relay agent option 82, the giaddr packet field must contain the virtual-network IP address of the relay interface, and this IP address must be reachable from the DHCP server in the underlay. Each VTEP that acts as a DHCP relay must have its virtual-network IP address installed using a route leaking mechanism as a route to the underlay and advertised to all underlay routers, including the spine switches.

Similarly, the DHCP server in the underlay VRF must be reachable from the client tenant VRF in the overlay. Configure a static route for the DHCP server subnet in the underlay default VRF, and leak the static route to the client tenant VRF in the overlay. This configuration sets up a bi-directional communication between the client and DHCP server across the virtual networks. The route-leaking configuration is not required if the VxLAN overlay subnet and underlay subnet are in same default VRF.

Configure DHCP relay on VTEPs

1. Configure the IP address of the virtual-network relay interface in the non-default tenant VRF as a static route in the default VRF.

   OS10(config)# ip route 10.10.0.2/32 interface virtual-network 10

2. Configure a static IP route to the DHCP server interface in the tenant VRF.

   OS10(config)# ip route vrf tenant01 40.1.1.0/24 interface vlan40

3. Configure DHCP relay on the virtual-network interface of the tenant VRF.

   OS10(config)# interface virtual-network 10
   OS10(conf-if-vn-10)# ip helper-address 40.1.1.1 vrf tenant01