Dell SmartFabric OS10 User Guide Release 10.5.3

VRRP Virtual IP as Server Override (sub option 11)

The server identifier (server ID) override sub-option allows the DHCP relay agent to specify a new value for the server ID option. This option inserted by the DHCP server in the BOOTPReply packet.

This sub-option allows the DHCP relay agent to act as a proxy to the actual DHCP server, such that the renew requests come to the relay agent rather than to the DHCP server directly.

The server ID override sub-option contains the IP address on the relay agent that is accessible from the client. Using this information, the DHCP client sends all the DHCP unicast packet (RENEW, RELEASE, DECLINE) to the relay agent. The relay agent adds all the appropriate option 82 sub-options and then forwards it to the configured helper-address for further processing.

In a VRRP scenario, the VRRP virtual IP is reachable from the DHCP clients; hence, it is used as the server-override option.

There is no option to configure any other interface IP as the server-override option.

VRRP is used for gateway redundancy in the LAN. All the hosts in the LAN use VRRP virtual IP as default route to forward packets towards the network.

In a DHCP relay use case, the DHCP client or host performs DISCOVER, OFFER, REQUEST, ACK (DORA) to acquire the IP address from the DHCP server. In normal scenarios, when the DHCP server sends the DHCP OFFER, it sets the server identifier option (option 54) to its IP address. This option is set, so that the DHCP clients use this IP address as destination address for any DHCP messages unicast to the DHCP server. The DHCP client sends unicast packets to RENEW, REQUEST, RELEASE, or DECLINE the IP address to the DHCP server.

In SmartFabric Services OS10 power switch, for DHCP relay across VRFs, in which DHCP client is in one VRF and DHCP server is residing in another VRF; the following option 82 sub options are used to achieve the functionality:

• Link Selection option (sub option 5)
• VSS option (sub option 151, sub option 152)
• Server Override option (sub option 11)
• Source interface configuration

In a Layer3 VLAN VLT scenario, for the DHCP unicast packet, one of the options is to use the any cast gateway IP address as the server-override option. This configuration enables the host to send the DHCP unicast packet with any cast gateway IP as the destination IP address. For use cases in which any cast gateway is not supported, VRRP protocol is used to achieve the functionality of the any cast gateway. VRRP and anycast gateway are mutually exclusive configurations. For example, in a SFD use case any cast gateway is not supported and hence VRRP is supported.

In a VRRP use case with DHCP client and DHCP server in different VRFs, you must use the VRRP virtual IP as the server-override option. This option enables the host or DHCP client connected in the LAN to send the DHCP unicast packets with virtual IP as the destination IP address. The DHCP relay agent uses the DHCP unicast packet with virtual IP as destination IP. It also appends the configured option 82 options before forwarding to the configured helper-address in the VRF for further processing.

Consider the scenario in the following figure:

In this scenario, in the VLT pairs (VLT Peer 1 and VLT Peer 2) VRRP is enabled and the virtual IP is configured to achieve gateway redundancy.

Alternatively, you can configure VLAN anycast gateway to achieve the gateway redundancy. VRRP and anycast gateway are mutually exclusive. The DHCP clients (Host 1, 2, 3) in VLAN 10 or VRF BLUE and DHCP clients (Host 1, 2, 3) in VLAN 20 or VRF RED use VRRP virtual IP as the default gateway.

The DHCP relay helper-address is configured in VLAN 10 and VLAN 20 with the VRF name Services (DHCP Server reachable VRF). For this use case, when server-override is enabled the VRRP virtual IP is used as server-override option when BOOTP Relay request is sent to the DHCP server. The DHCP server sets the server-override option in the server identifier option in the DHCP OFFER packet. The DHCP client uses the VRRP virtual IP or server-override option as destination IP in the DHCP unicast packet. The DHCP relay forwards the packet to the DHCP server for further processing.