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DHCP Relay Design and Implementation

For an overview of the DHCP Relay implementation in this design, see the DHCP Relay section in Data Center Fabric Blueprint Architecture Components.

DHCP Relay was validated on centrally-routed bridging overlays (see Centrally-Routed Bridging Overlay Design and Implementation) in use cases like those described below. DHCP Relay has also been validated with edge-routed bridging overlays (see Edge-Routed Bridging Overlay Design and Implementation) starting in Junos OS Releases 18.4R2-S5 and 19.1R1. You can refer to Configure a DHCP Relay in EVPN-VXLAN Fabric Architecture for a network configuration example of a DHCP Relay use case with an edge-routed bridging overlay.

Figure 1 provides a sample DHCP server and client placement in this design.

Figure 1: DHCP Server and Client SetupDHCP Server and Client Setup

This section covers the following DHCP Relay scenarios:

Enabling DHCP Relay: DHCP Client and Server in Same VLAN and Connected to Same Leaf Device

Figure 2 shows how DHCP traffic might traverse the overlay network when the DHCP client and server connect to access interfaces on the same leaf device and are in the same VLAN.

Figure 2: DHCP Relay—Same Leaf Device and Same VLANDHCP Relay—Same Leaf Device and Same VLAN

DHCP traffic is handled like any other intra-VLAN traffic in this scenario. It could also have been passed in VNI_10000 on leaf device 10 or 11. No additional configuration of the leaf or spine devices is required to support this traffic exchange.

Enabling DHCP Relay: DHCP Client and Server in Same VLAN and Connected to Different Leaf Devices

Figure 3 shows how DHCP traffic might be forwarded when the DHCP client and server connect to access interfaces on non-connected leaf devices that have interfaces in the same VLAN.

Figure 3: DHCP Relay—Different Leaf Device and Same VLANDHCP Relay—Different Leaf Device and Same VLAN

In this scenario, the DHCP traffic remains in VNI_10000 and is passed over the topology using a VXLAN tunnel. No additional configuration of the leaf or spine devices is required to support this traffic exchange.

Enabling DHCP Relay: DHCP Client and Server in Different VLANs

Figure 4 shows how DHCP traffic is forwarded when the DHCP client and server connect to access interfaces in different VLANs.

Figure 4: DHCP Relay—Different VLANsDHCP Relay—Different VLANs

When the DHCP client and server are in different VNI, DHCP traffic is forwarded to a spine device. The DHCP traffic is forwarded between VNIs via the IRB interfaces on the spine device and then passed to the destination DHCP client or server.

The IRB interfaces on the spine devices must be configured to support DHCP Relay.

To prepare the network to support DHCP relay when the DHCP client and server are in different VLANs:

  1. Ensure the centrally-routed bridging overlay is configured. See Centrally-Routed Bridging Overlay Design and Implementation.
  2. Enable DHCP relay in a routing instance with the forward only option. The forward only option ensures that DHCP packets are forwarded on the switch but that no DHCP server client bindings are created.

    Spine Device:

  3. Create and activate the DHCP relay server group. The DHCP relay server group include one or more DHCP servers—individually identified by IP address—and a user-defined name for the servers.

    In this reference design, one DHCP server—10.1.0.211—is assigned into a DHCP server group named DHCP_SERVER_GROUP_1.

    Spine Device:

  4. Associate the server group with the IRB interfaces on the spine devices.

    In this topology, irb.100 and irb.200 relay DHCP traffic between VNI_10000 and VNI_20000.

    Spine Devices:

  5. Confirm that the DHCP hosts received an IP address by reviewing the ARP table on the spine device.

DHCP Relay — Release History

Table 1 provides a history of all of the features in this section and their support within this reference design.

Table 1: DHCP Relay Release History

Release

Description

19.1R2

QFX10002-60C and QFX5120-32C switches running Junos OS Release 19.1R2 and later releases in the same release train and serve as spine devices also support all features documented in this section.

We have not validated DHCP Relay between VLANs in edge-routed bridging overlays for this reference design.

18.4R2

QFX5120-48Y switches running Junos OS Release 18.4R2 and later releases in the same release train support all features documented in this section.

18.1R3-S3

QFX5110 switches running Junos OS Release 18.1R3-S3 and later releases in the same release train support all features documented in this section.

17.3R3-S1

All spine devices in the reference design that support Junos OS Release 17.3R3-S1 and later releases in the same release train also support all features documented in this section.

We have not validated DHCP Relay between VLANs in edge-routed bridging overlays for this reference design.

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