Users Guide

In a VRRP scenario, the VRRP virtual IP is reachable from the DHCP clients; hence, it is used as the server-override option.
NOTE: In the current implementation, SmartFabric Services OS10 supports enabling or disabling of the server-override
option. If the server-override option is enabled, the anycast gateway is used as the server-override option. VRRP and
anycast gateway are mutually exclusive. Hence, for VRRP to support the server-override option, VRRP virtual IP 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:
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System management