DHCP server runs as a service (service router) in the edge nodes of an NSX Edge cluster.

Isolated segments that are not connected to a gateway can use only a Local DHCP server. Segments that are connected to a gateway on the downlink interface can use either a Local DHCP server, DHCP Relay, or Gateway DHCP server.

Regardless of whether a segment uses a Local DHCP server or a Gateway DHCP server, DHCP server always runs as a service router in the edge transport nodes of an edge cluster. If the segment uses a Local DHCP server, the DHCP service is created in the edge cluster that you specified in the DHCP profile. However, if the segment uses a Gateway DHCP server, the edge cluster in which the DHCP service is created depends on the combination of the following factors:
  • Is an edge cluster specified in the gateway?
  • Is an edge cluster specified in the DHCP profile of the gateway?
  • Is the edge cluster in the gateway and in the DHCP profile same or different?
  • Is the tier-1 routed segment connected to a tier-0 gateway?

The following scenarios explain how the edge cluster is selected for creating the DHCP service.

Scenario 1: Isolated Segment Uses Local DHCP Server

Scenario Description:
  • An edge cluster (Cluster1) is created with four edge nodes: N1, N2, N3, N4.
  • A segment with None connectivity is added in the overlay transport zone.
  • Segment uses a Local DHCP server, by default.
The DHCP server profile configuration is as follows:
  • Profile Type: DHCP Server
  • Edge Cluster: Cluster1
  • Preferred Edges: None

In this scenario, any two edge nodes from Cluster1 are autoallocated to create the DHCP service, and DHCP high availability (HA) is automatically configured. One of the edge nodes in Cluster1 runs in active mode and the other edge runs in passive mode.

Note:
  • If you select two preferred edge nodes in the DHCP profile, the edge node that is added first becomes the active edge. The second edge node takes the passive role.
  • If you select only one preferred edge node in the DHCP profile, DHCP HA is not configured.

Scenario 2: Tier-1 Routed Segment Uses Gateway DHCP and Different Edge Clusters in Gateway and DHCP Profile

Consider that you have two edge clusters in your network (Cluster1 and Cluster2). Both clusters have four edge nodes each:
  • Cluster1 edge nodes: N1, N2, N3, N4
  • Cluster2 edge nodes: N5, N6, N7, N8
Scenario Description:
  • Segment is connected to a tier-1 gateway.
  • Tier-1 gateway is not connected to a tier-0 gateway.
  • DHCP server profile in the tier-1 gateway uses Cluster1.
  • Tier-1 gateway uses Cluster2.
  • Segment is configured to use the Gateway DHCP server.
The DHCP server profile in the tier-1 gateway has the following configuration:
  • Profile Type: DHCP Server
  • Edge Cluster: Cluster1
  • Preferred Edges: N1,N2 (added in the given sequence)
The tier-1 gateway configuration is as follows:
  • Edge Cluster: Cluster2
  • Preferred Edges: N5,N6 (added in the given sequence)

In this scenario, DHCP service runs on the edge nodes of Cluster2. As Cluster2 contains multiple edge nodes, DHCP HA is autoconfigured. However, the preferred edges N5 and N6 on the gateway are ignored for DHCP HA. Any two nodes from Cluster2 are randomly autoallocated for DHCP HA.

This scenario also applies when the segment is directly connected to a tier-0 gateway, and there is no tier-1 gateway in your network topology.

Caution: You can change the edge cluster on the Gateway DHCP server after the DHCP server is created. However, this action causes all the existing DHCP leases that are assigned to the DHCP clients to be lost.
To summarize, the main points of this scenario are as follows:
  • When you use a Gateway DHCP server and set different edge clusters in the gateway DHCP profile and tier-1 gateway, then DHCP service is always created in the edge cluster of the gateway.
  • The edge nodes are randomly allocated from the edge cluster of the tier-1 gateway for DHCP HA configuration.
  • If no edge cluster is specified in the tier-1 gateway, the edge cluster in the DHCP profile of the tier-1 gateway (Cluster1) is used to create the DHCP service.

Scenario 3: Tier-1 Routed Segment Uses Local DHCP Server and Different Edge Clusters in Gateway and DHCP Profile

In this scenario, a segment is connected to a tier-1 gateway, but you use a Local DHCP server on the segment. Consider that you have three edge clusters in your network (Cluster1, Cluster2, Cluster 3). Each cluster has two edges nodes each.
  • Cluster1 edge nodes: N1, N2
  • Cluster2 edge nodes: N3, N4
  • Cluster3 edge nodes: N5, N6
Scenario Description:
  • Segment is connected to a tier-1 gateway.
  • Tier-1 gateway is connected to a tier-0 gateway (optional).
  • DHCP profile on the gateway uses Cluster1.
  • Gateway uses Cluster2.
  • Segment is configured to use Local DHCP server.
  • Local DHCP server profile uses Cluster3.
The DHCP server profile on the gateway is as follows:
  • Profile Name: ProfileX
  • Profile Type: DHCP Server
  • Edge Cluster: Cluster1
  • Preferred Edges: N1,N2 (added in the given sequence)
The tier-1 gateway configuration is as follows:
  • Edge Cluster: Cluster2
  • Preferred Edges: N3,N4 (added in the given sequence)
The profile on the Local DHCP server is as follows:
  • Profile Name: ProfileY
  • Profile Type: DHCP Server
  • Edge Cluster: Cluster3
  • Preferred Edges: N5,N6 (added in the given sequence)

In this scenario, because the segment is configured to use a Local DHCP server, the edge cluster (Cluster2) in the connected tier-1 gateway is ignored to create the DHCP service. DHCP service runs in the edge nodes of Cluster3 (N5, N6). DHCP HA is also configured. N5 becomes the active edge node and N6 becomes the standby edge.

If no preferred nodes are set in Cluster3, any two nodes from this cluster are autoallocated for creating the DHCP service and configuring DHCP HA. One of the edge nodes becomes an active edge and the other node becomes the standby edge. If only one preferred edge node is set in Cluster3, DHCP HA is not configured.

This scenario also applies when the segment is directly connected to a tier-0 gateway, and there is no tier-1 gateway in your network topology.

Scenario 4: Tier-1 Routed Segment Uses Gateway DHCP and Same Edge Clusters in Gateway and DHCP Profile

Consider that you have a single edge cluster (Cluster1) in your network with four edge nodes: N1, N2, N3, N4.

Scenario Description:
  • Segment is connected to a tier-1 gateway.
  • Tier-1 gateway is connected to a tier-0 gateway (optional)
  • Gateway and DHCP profile on the gateway use the same edge cluster (Cluster1).
  • Segment is configured to use Gateway DHCP server.
The DHCP server profile on the gateway is as follows:
  • Profile Type: DHCP Server
  • Edge Cluster: Cluster1
  • Preferred Edges: N1,N2 (added in the given sequence)
The tier-1 gateway configuration is as follows:
  • Edge Cluster: Cluster1
  • Preferred Edges: N3,N4 (added in the given sequence)

In this scenario, as the gateway DHCP profile and gateway use a similar edge cluster (Cluster1), DHCP service is created in the preferred edge nodes N1 and N2 of the gateway DHCP profile. The preferred edge nodes N3 and N4 that you specified in the connected tier-1 gateway are ignored for creating the DHCP service.

If no preferred edges are set in the DHCP profile, any two nodes from Cluster1 are autoallocated for creating the DHCP service and configuring DHCP HA. One of the edge nodes becomes an active edge and the other edge becomes the standby edge.

To summarize, the main points of this scenario are as follows:
  • When you use a Gateway DHCP server and specify similar edge clusters in the DHCP profile and connected gateway, then DHCP service is created in the preferred edge nodes of the DHCP profile.
  • The preferred edges nodes specified in the connected gateway are ignored.

Scenario 5: Tier-1 Routed Segment is Connected to Tier-0 Gateway and No Edge Cluster is Set in Tier-1 Gateway

In this scenario, a segment is connected to a tier-1 gateway, and the tier-1 gateway is connected to a tier-0 gateway. Consider that you have three edge clusters in your network (Cluster1, Cluster2, Cluster 3). Each cluster has two edges nodes each.
  • Cluster1 edge nodes: N1, N2
  • Cluster2 edge nodes: N3, N4
  • Cluster3 edge nodes: N5, N6
Scenario Description:
  • Segment is directly connected to a tier-1 gateway.
  • Tier-1 gateway is connected to a tier-0 gateway.
  • DHCP server profile is specified on both tier-1 and tier-0 gateways.
  • DHCP profile on tier-1 gateway uses Cluster1.
  • DHCP profile on tier-0 gateway uses Cluster2.
  • No edge cluster is selected in tier-1 gateway.
  • Tier-0 gateway uses Cluster3.
  • Segment is configured to use a Gateway DHCP server.

In this scenario, because the tier-1 gateway has no edge cluster specified, NSX-T Data Center falls back on the edge cluster of the connected tier-0 gateway. DHCP service is created in the edge cluster of tier-0 gateway (Cluster3). Any two edge nodes from this edge cluster are autoallocated for creating the DHCP service and configuring DHCP HA.

To summarize, the main points of this scenario are as follows:
  • When a tier-1 gateway has no edge cluster specified, NSX-T Data Center falls back on the edge cluster of the connected tier-0 gateway to create the DHCP service.
  • If no edge cluster is detected in the tier-0 gateway, DHCP service is created in the edge cluster of the tier-1 gateway DHCP profile.