Users Guide

The server ID override suboption carries the virtual anycast gateway IP (which is the IP address on the relay agent) that is
accessible from the client. The DHCP client uses this information to send all renew and release request packets to the relay
agent. The relay agent adds all of the appropriate suboptions and then forwards the renew and release request packets to the
original DHCP server.
If configured, the server identifier (ID) override suboption carries virtual anycast gateway IP. Otherwise, the option is not sent in
the DHCP request.
Link selection suboption- Sub-option 5(0x5)
The link selection suboption provides a mechanism to separate the subnet or link on which the DHCP client resides from the
gateway address (giaddr). Use this gateway address to communicate with the relay agent by the DHCP server. The relay
agent sets the suboption to the correct subscriber IP and the DHCP server uses that value to assign an IP address from that
subnet rather than the giaddr value.
NOTE: The DHCP server allocates the IP address based on the link-selection suboption. If the link-selection is not present,
the giaddr option is used to allocate IP address.
The DHCPv4 relay agent must support link selection sub-option 5 based on the following order of precedence:
● If the IP address configured on interface, use the interface IP as the subnet IP for sub-option 5.
● If you configure the virtual anycast gateway IP address on interface, use the virtual anycast gateway IP as the subnet IP for
sub-option 5.
DHCP source-Interface
If the client-connected interface is unnumbered, the server may not be able to reach the relay agent. This feature manually
configures the interface for the relay agent to use as the source IP address for messages relayed to the DHCP server, which is
used by the server to send the reply. This configuration allows the network administrator to specify a stable IP address (such as
a Loopback interface). The specified interface IP address is used to fill the giaddr by the DHCP relay agent.
NOTE: Dell Technologies recommends configuring different DHCP relay source IP addresses for the VLT peers.
DHCPv4 Virtual subnet selection option - Sub-option 151(0x97) and DHCPv4 virtual subnet selection control -
Sub-option 152(0x98)
This sub-option conveys DHCP client VRF-related information to the DHCP server in an L3-VRF-Lite and VXLAN-EVPN-multi-
tenant environment. Suboption 152 determines whether the DHCP server understood the VSS sub-option 151 or not. The VRF
identifier suboption is used by the relay agent to tell the DHCP server the VRF for every DHCP request it passes on to the
DHCP server.
The VRF identifier suboption contains the VRF ID or VRF name configured on the incoming interface to which the client is
connected. The VXLAN VTEP acts as a relay agent, providing DHCP relay services in a multi-tenant VXLAN environment.
The network element that contains the relay agent captures the VRF association of the DHCP client connected interface and
includes this information in the relay agent information option of the DHCP packet.
Restrictions and Limitations
● This feature is not supported on PVLAN.
● DHCP relay options do not work if Option-82 is disabled globally or at the interface level.
● The server-override option is supported only on VXLAN VXLAN-VLT scenarios.
● The DHCP relay source-interface configuration at the VRF level may have problem in batch-apply. This IP VRF context
comes in the beginning of the running configuration even before the Loopback interface configuration. When the IP VRF
configuration plays, Loopback is not present and it will cause an error. This is similar to the existing behavior with update-
source interface command.
● With server override enabled, the DHCP relay drops further packets from the DHCP client if there is change in the anycast
gateway IP address. This forces the client to restart the discovery process.
System operation and behavior
A combination of the Option-82 sub-options (suboption-5, suboption-11, suboption-151) are used in the multi-tenant EVPN
environment, where the relay agent supports multiple clients on different VPNs. Many of these clients from different VPNs can
have identical IP addresses (as they are in different L2 domains or different L3 domains; for example, VRFs).
A DHCP server that provides service to DHCP clients on different VPNs must be informed of the VPN identity in which each
client resides. You can configure the OS10 DHCP relay agent to provide information about the DHCP client-to-VPN association
in options added to the DHCP packets that it relays to the DHCP server.
When the DHCP client, server, and source-interface belongs to different VRFs, you must configure route leaking between the
VRFs to establish communication across VRFs.
DHCP relay options work only with default Option-82 settings.
268
System management