Dell SmartFabric OS10 User Guide Release 10.5.3

DHCP relay without route leaking

This feature enables you to support DHCP relay across VRFs without route leaking. DHCP relay with DHCP client and DHCP server in different VRFs requires route reachability for the DHCP server. You can achieve route reachability using the route leaking method.

To enable DHCP relay across VRFs without route leaking for SFD with open config model, the existing helper-address configuration is modified to allow any VRF name. For this configuration to work, you must first configure a helper-address reachable VRF.

Consider the scenario depicted in the following figure:

In this scenario the DHCP clients (Host 1, 2, 3) are part of the VRF Blue. The DHCP packet sent by these DHCP clients reach the DHCP relay in VLAN 10 in the VRF Blue. In the VLAN 10, you must configure the helper-address with the DHCP server reachable VRF services. DHCP relay forwards the DHCP client packets to the DHCP server without route leaking by forwarding it in the services VRF. The DHCP relay appends the option 82 and its following sub options:

• Link selection option (sub option 5) set to the IP address of the VLAN 10.
• Server-override option (sub option 11) set to the IP address of the VRRP virtual IP or anycast gateway IP.
• VSS option (sub option 151 and sub option 152) set to the VRF Blue.
• You must configure VLAN 20 on interface Ethernet 1/1/4 as source interface, which sets the giaddr field in the BOOTP relay request packet towards the DHCP server.

The BOOTP relay reply packet from the DHCP server contains the destination IP address set to the IP address of the source interface (VLAN 20). This interface is forwarded to the DHCP client by the DHCP relay in VRF Blue after removing the DHCP option 82 and its relay sub options.

The DHCP server allocates the IP address to the DHCP client based on the link selection and VSS options. The DHCP unicast packet from the DHCP client is sent to the server-override IP. After appending the option 82 and its sub options, DHCP relay forwards this packet to the DHCP server.