Dell SmartFabric OS10 User Guide Release 10.5.3

DHCP relay agent options

When enabled on VLANs, DHCP Option-82 is inserted by the DHCP relay agent. When a network device, such as a DHCP client, is connected to the VLAN on an untrusted interface sends a DHCP request, the relay agent inserts information about the client network location into the packet header (the options field of the DHCP packet is Option-82 circuitid) of that request.

The relay agent then sends the request to the DHCP server. The DHCP server reads the Option-82 information in the packet and uses it to determine the IP address or another parameter assignments for the client. After receiving the response from the DHCP server, the relay agent strips the DHCP Option-82 and forwards it to the client.

Use Option-82 to uniquely identify the domain in which the DHCP client is connected. Option-82 carries two sub-options: circuit-id and remote-id (for snooping-enabled VLANs).

• circuit-id: This sub option contains the VLAN and port information of the DHCP client. VLAN id and Port name are used for this option. The circuit- id is added in the <VLANID>-<INTERFACE_NAME> format. For example: vlan100-eth1/1/1. The DHCP relay packet is appended to the circuit id and the circuit id becomes the interface port number.
• remote-id: This sub option contains the system identification. System MAC address is used for this sub-option. For example: 00:04:89:76:62:78, for dhcp snooping.

Additionally, DHCPv4 Option-82 sub-options include:

• Server ID override suboption Sub-option 11(0xb)
• Link selection suboption- Sub-option 5(0x5)
• DHCPv4 virtual subnet selection option - Sub-option 151(0x97)
• DHCPv4 virtual subnet selection control - Sub-option 152(0x98)

• source-interface CLI for relay agents. The gateway address (giaddr) field carries the source interface address.

Server ID override suboption sub-option 11(0xb)

The server ID override suboption allows the DHCP relay agent to specify a new value for the server ID option, which is inserted by the DHCP server in the reply packet. This suboption allows the DHCP relay agent to act as the proxy for the DHCP server so that the renew requests comes to the relay agent rather than to the DHCP server directly.

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.

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.

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.