Users Guide

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.
In VxLAN symmetric and asymmetric IRB scenarios, Dell Technologies recommends having the same relay configurations at the
Global and Interface level on the VTEP OS10 devices.
Dell Technologies recommends enabling the DHCPv4 virtual subnet selection and link selection options along with the server ID
override option. Enabling DHCPv4 options without the server ID override option might not work when the DHCPv4 server
renews or releases the client IP address that is based on these options. The server ID override option is supported only on VLAN
or VXLAN interfaces as the virtual-router (virtual router anycast gateway IP) configuration is available only on VLAN or VXLAN
interfaces and the same is used to fill the server ID override option.
Use Case 1: Source-interface CLI link selection and server-override
suboptions
In the following example, the DHCP client is connected on eth 1/1/1.
System management
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