Use this reference documentation to understand the various considerations that you must keep in mind while configuring the NSX DHCP service and to obtain a detailed guidance about the configuration settings on the Set DHCP Config page.

The following note mentions the NSX DHCP configuration types that are supported or not supported based on how overlay or VLAN segments are connected:

Note:
  • On an isolated segment that is not connected to a gateway, only Segment DHCP server is supported.
  • Segments that are configured with an IPv6 subnet can have either a Segment DHCPv6 server or a DHCPv6 relay. Gateway DHCPv6 server is not supported.
  • If a segment contains an IPv4 subnet and an IPv6 subnet, you can configure both DHCPv4 and DHCPv6 servers on the segment.
  • DHCPv4 relay is supported on a VLAN segment through the service interface of a tier-0 or tier-1 gateway. Only one DHCPv4 relay service is supported on a VLAN segment.
  • For a VLAN segment requiring a DHCP server, only Segment DHCP server is supported. Gateway DHCP server is not supported on a VLAN segment.
When a Segment DHCP server or DHCP Relay is configured on a VLAN segment, one of the following configurations is required:
  • For VDS or VSS, Forged Transmits must be set to Accept.
  • For NSX segments or VDS 6.6 and later, MAC Learning must be set to Enabled.
In vSphere 7.0 or later, the Forged Transmits option is set to Reject, by default. To enable this option on the host, do the following steps:
  1. Log in to the vSphere Client UI with admin privileges.
  2. Go to Hosts and Clusters and click a host in the cluster.
  3. Navigate to Configure > Networking > Virtual Switches, and then click Edit.
  4. On the Edit Settings window, click Security. From the Forged Transmits drop-down menu, select Accept.

To learn about Forged Transmits, see the vSphere Security documentation.

Important: After a segment has DHCP service configured on it, some restrictions and caveats apply on changing the connectivity of the segment. For more information, see Scenarios: Impact of Changing Segment Connectivity on NSX DHCP.

The following sections provide guidance about the configuration settings on the Set DHCP Config page.

DHCP Type

  • When a segment is connected to a gateway, Segment DHCP server is selected by default. If needed, you can select Gateway DHCP server or DHCP Relay from the drop-down menu.
  • If you select the DHCP type as Gateway DHCP server, the DHCP profile that is attached to the gateway is autoselected. The name and server IP address are fetched automatically from that DHCP profile and displayed in a read-only mode.
  • For an isolated segment, which is not connected to a gateway, Segment DHCP server is selected by default.

DHCP Profile

  • When you are configuring a Segment DHCP server or a DHCP Relay on the segment, you must select a DHCP profile from the drop-down menu. If no profiles are available in the DHCP Profile drop-down menu, click Actions menu and create a DHCP profile. After the profile is created, it is automatically attached to the segment.
  • When a segment is using a Gateway DHCP server, ensure that an edge cluster is selected either in the gateway, or DHCP server profile, or both. If an edge cluster is unavailable in either the profile or the gateway, an error message is displayed when you save the segment.
  • When a segment is using a Segment DHCP server, ensure that the DHCP server profile contains an edge cluster. If an edge cluster is unavailable in the profile, an error message is displayed when you save the segment.

IPv4 Server or IPv6 Server Settings

This section explains the configuration settings in the IPv4 Server tab page and the IPv6 Server tab page.

DHCP Server Address
  • If you are configuring a Segment DHCP server, server IP address is required. A maximum of two server IP addresses are supported. One IPv4 address and one IPv6 address. For an IPv4 address, the prefix length must be <= 30, and for an IPv6 address, the prefix length must be <= 126. The server IP addresses must belong to the subnets that you have specified in this segment. The DHCP server IP address must not overlap with the IP addresses in the DHCP ranges and DHCP static binding. The DHCP server profile might contain server IP addresses, but these IP addresses are ignored when you configure a Segment DHCP server on the segment.
  • After a Segment DHCP server is created, you can edit the server IP addresses on the Set DHCP Config page. However, the new IP addresses must belong to the same subnet that is configured in the segment.
  • If you are configuring a Gateway DHCP server, the DHCP Server Address text box is not editable. The server IP addresses are fetched automatically from the DHCP profile that is attached to the connected gateway.
  • The Gateway DHCP server IP addresses in the DHCP server profile can be different from the subnet that is configured in the segment. In this case, the Gateway DHCP server connects with the IPv4 subnet of the segment through an internal relay service, which is autocreated when the Gateway DHCP server is created. The internal relay service uses any one IP address from the subnet of the Gateway DHCP server IP address.
  • The IP address used by the internal relay service acts as the default gateway on the Gateway DHCP server to communicate with the IPv4 subnet of the segment.
  • After a Gateway DHCP server is created, you can edit the server IP addresses in the DHCP profile of the gateway. However, you cannot change the DHCP profile that is attached to the gateway. When a DHCP server profile is used in your network, preferably avoid editing the server IP addresses in the DHCP server profile. It might cause a failure while renewing or releasing the IP addresses that are leased to the DHCP clients.
DHCP Ranges
  • IP ranges, CIDR subnet, and IP addresses are allowed. IPv4 addresses must be in a CIDR /32 format, and IPv6 addresses must be in a CIDR /128 format. You can also enter an IP address as a range by entering the same IP address in the start and the end of the range. For example, 172.16.10.10-172.16.10.10.
  • IP addresses in the DHCP ranges must belong to the subnet that is configured on the segment. That is, DHCP ranges cannot contain IP addresses from multiple subnets.
  • IP ranges must not overlap with the DHCP server IP address and the DHCP static binding IP addresses.
  • IP ranges in the DHCP IP pool must not overlap each other.
  • Number of IP addresses in any DHCP range must not exceed 65536.
  • The following types of IPv6 addresses are not permitted in DHCP for IPv6 ranges:
    • Link Local Unicast addresses (FE80::/64)
    • Multicast addresses (FF00::/8)
    • Unspecified address (0:0:0:0:0:0:0:0)
    • Address with all F (F:F:F:F:F:F:F:F)
Caution: After a DHCP server is created, you can update existing ranges, append new IP ranges, or delete existing ranges. However, it is a good practice to avoid deleting, shrinking, or expanding the existing IP ranges. For example, do not try to combine multiple smaller IP ranges to create a single large IP range. When you modify existing ranges after the DHCP service is running, it might cause the DHCP clients to lose network connection and result in a temporary traffic disruption.
Excluded Ranges (Only for DHCPv6)

Enter IPv6 addresses or a range of IPv6 addresses that you want to exclude for dynamic IP assignment to DHCPv6 clients.

In IPv6 networks, the DHCP ranges can be large. Sometimes, you might want to reserve certain IPv6 addresses, or multiple small ranges of IPv6 addresses from the large DHCP range for static binding. In such situations, you can specify excluded ranges.

Lease Time

Default value is 86400 seconds. Valid range of values is 60–4294967295.

Preferred Time (Only for DHCPv6)

Preferred time is the length of time that a valid IP address is preferred. When the preferred time expires, the IP address becomes deprecated. If no value is entered, preferred time is autocalculated as (lease time * 0.8).

Lease time must be > preferred time.

Valid range of values is 60–4294967295. Default is 69120 seconds.

DNS Servers

A maximum of two DNS servers are permitted. When not specified, no DNS is assigned to the DHCP client.

Domain names (Only for DHCPv6)

One or more domain names are supported.

DHCPv4 server configuration automatically fetches the domain name that you specified in the segment configuration.

SNTP Servers (Only for DHCPv6)

A maximum of two SNTP servers are permitted.

DHCPv6 server does not support NTP.

DHCP Options (Only for DHCPv4)

DHCP Options for IPv6 are not supported.

Each classless static route option in DHCP for IPv4 can have multiple routes with the same destination. Each route includes a destination subnet, subnet mask, next hop router. For information about classless static routes in DHCPv4, see RFC 3442 specifications. You can add a maximum of 127 classless static routes on a DHCPv4 server.

In addition to the Generic Option 121 (classless static route), NSX supports other Generic Options that are described in the following table. The Generic Options, which are not listed in this table are also accepted without any validation, but they do not take effect.

Table 1. Supported Generic Options
Code Name Value Type Example Value

2

Time Offset

Integer - seconds offset from UTC

Allowed values: -43200–43200

Maximum items: 1

28800

13

Boot File Size

Number of blocks. One block is 512 bytes.

Integer values: 1–65535

Maximum items: 1

1385

19

Forward On/Off

IP forwarding

Allowed values: [0, 1]

1 for on, 0 for off

Maximum items: 1

0

26

MTU Interface

MTU for a given interface.

Allowed values: 68–65535

Maximum items: 1

9600

28

Broadcast Address

IP address

Maximum items: 1

10.10.10.255

35

ARP Timeout

Integer (seconds)

Allowed values: 0–4294967295

360

40

NIS Domain

Text

Maximum: 255 characters

vmware.com

41

NIS Servers

IP addresses in a preferred order

Maximum items: 63

10.10.10.10

42

NTP Servers

IP addresses in a preferred order

Maximum items: 63

10.10.10.11

44

NETBIOS Name Server

IP addresses in a preferred order

Maximum items: 63

10.10.10.12

45

NETBIOS Dist Server

IP addresses in a preferred order

Maximum items: 63

10.10.10.13

46

NETBIOS Node Type

Integer encoding of node type

Allowed values: [1, 2, 4, 6]

Maximum items: 4

1 = B-node - broadcast no WINS

2 = P-node - WINS only

4 = M-node - broadcast then WINS

8 = H-node - WINS then broadcast

2

47

NETBIOS Scope

String encoded according to RFC 1001/1002

Maximum: 255 characters

58

Renewal Time

N/A - based on the lease time between 0–4294967295

Maximum items: 1

300

59

Rebinding Time

N/A - based on the lease time between 0–4294967295

Maximum items: 1

300

64

NIS+ Domain Name

Text (domain name)

vmware.com

65

 NIS+ Server Address

IP addresses in a preferred order

10.10.10.10

66

Server Name

Text (server domain name)

Maximum: 255 characters

10.10.10.253

67

Bootfile Name

Text (file name)

Maximum: 255 characters

/tftpboot/pxelinux/pxelinux.bin

117

Name Service Search

Not natively supported with API

Allowed values: [0, 6, 41, 44, 65]

Maximum items: 5

6

119

Domain Search

One or more domain names. Each domain name must be enclosed in quotes and separated by commas.

vmware.com

150

TFTP server address

IP address

10.10.10.10

209

PXE Configuration File

Maximum: 255 characters

configs/common

210

PXE Path Prefix

Maximum: 255 characters

/tftpboot/pxelinux/files/

211

PXE Reboot Time

Allowed values: 0–4294967295

1800

DHCP Static Bindings

In a typical network environment, you have VMs that run services, such as FTP, email servers, application servers, and so on. You might not want the IP address of these VMs to change in your network. In this case, you can bind a static IP address to the MAC address of each VM (DHCP client). The static IP address must belong to the subnet (if any) that is configured on the segment, and it must not overlap with the DHCP IP ranges and DHCP server IP address.

DHCP static bindings are supported when you are configuring either a Segment DHCP server or a Gateway DHCP server on the segment. When a segment is using a DHCP Relay, you cannot configure static bindings.

On a DHCP for IPv4 server, static bindings are supported regardless of whether the segment uses a Segment DHCP or a Gateway DHCP configuration. On a DHCP for IPv6 server, static bindings are supported only when the segment uses a Segment DHCP configuration.

Static Binding Options Common to DHCPv4 and DHCPv6 Server

The following table describes the static binding options that are common to DHCP for IPv4 and DHCP for IPv6 servers.

Option Description
Name

Enter a unique display name to identify each static binding. The name must be limited to 255 characters.
MAC Address

Required. Enter the MAC address of the DHCP client to which you want to bind a static IP address.

The following validations apply to MAC address in static bindings:
  • MAC address must be unique in all the static bindings on a segment that uses a Segment DHCP server.
  • MAC address must be unique in all the static bindings across all the segments that are connected to the gateway and which use the Gateway DHCP server.
For example, consider that you have 10 segments connected to a tier-1 gateway. You use a Gateway DHCP server for four segments (Segment1 to Segment4), and a Segment DHCP server for the remaining six segments (Segment5 to Segment10). Assume that you have a total of 20 static bindings across all the four segments (Segment1 to Segment4), which use the Gateway DHCP server. In addition, you have five static bindings in each of the other six segments (Segment5 to Segment10), which use a Segment DHCP server. In this example:
  • The MAC address in each of the 20 static bindings must be unique across all the segments (Segment1 to Segment4) that use the Gateway DHCP server.
  • The MAC address in the five static bindings must be unique on each segment (Segment5 to Segment10) that use a Segment DHCP server.
IP Address
  • Required for IPv4 static binding. Enter a single IPv4 address to bind to the MAC address of the client.
  • Optional for IPv6 static binding. Enter a single Global Unicast IPv6 address to bind to the MAC address of the client.

When no IPv6 address is specified for static binding, Stateless Address Autoconfiguration (SLAAC) is used to auto-assign an IPv6 address to the DHCPv6 clients. Also, you can use Stateless DHCP to assign other DHCP configuration options, such as DNS, domain names, and so on, to the DHCPv6 clients.

For more information about Stateless DHCP for IPv6, read the RFC 3376 specifications.

The following types of IPv6 addresses are not permitted in IPv6 static binding:
  • Link Local Unicast addresses (FE80::/64 )
  • Multicast IPv6 addresses (FF00::/8)
  • Unspecified address (0:0:0:0:0:0:0:0)
  • Address with all F (F:F:F:F:F:F:F:F)

The static IP address must belong to the subnet (if any) that is configured on the segment, and it must be outside the DHCP ranges that you have configured on the segment.
Lease Time

Optional. Enter the amount of time in seconds for which the IP address is bound to the DHCP client. When the lease time expires, the IP address becomes invalid and the DHCP server can assign the address to other DHCP clients on the segment.

Valid range of values is 60–4294967295. Default is 86400.
Description

Optional. Enter a description for the static binding.
Tags

Optional. Add tags to label static bindings so that you can quickly search or filter bindings, troubleshoot and trace binding-related issues, or do other tasks.

For more information about adding tags and use cases for tagging objects, see NSX Tags.
Static Binding Options (Only in DHCPv4 Server)

The following table describes the static binding options that are available only in a DHCP for IPv4 server.

DHCP For IPv4 Option Description
Gateway Address

Enter the default gateway IP address that the DHCP for IPv4 server must provide to the DHCP client.
Host Name

Enter the host name of the DHCP for IPv4 client so that the DHCPv4 server can always bind the client (host) with the same IPv4 address each time.

The host name must be limited to 63 characters.

The following validations apply to host name in static bindings:
  • Host name must be unique in all the static bindings on a segment that uses a Segment DHCP server.
  • Host name must be unique in all the static bindings across all the segments that are connected to the gateway and which use the Gateway DHCP server.
For example, consider that you have 10 segments connected to a tier-1 gateway. You use a Gateway DHCP server for four segments (Segment1 to Segment4), and a Segment DHCP server for the remaining six segments (Segment5 to Segment10). Assume that you have a total of 20 static bindings across all the four segments (Segment1 to Segment4), which use the Gateway DHCP server. In addition, you have five static bindings in each of the other six segments (Segment5 to Segment10), which use a Segment DHCP server. In this example:
  • The host name in each of the 20 static bindings must be unique across all the segments (Segment1 to Segment4) that use the Gateway DHCP server.
  • The host name in the five static bindings must be unique on each segment (Segment5 to Segment10) that use a Segment DHCP server.
DHCP Options

Optional. Click Set to configure DHCP for IPv4 Classless Static Routes and other Generic Options.
Some additional notes for DHCPv4 static binding:
  • IPv4 static bindings automatically inherit the domain name that you configured on the segment.
  • To specify DNS servers in the static binding configuration, add the Generic Option (Code 6 - DNS Servers).
  • To synchronize the system time on DHCPv4 clients with DHCPv4 servers, use NTP. DHCP for IPv4 server does not support SNTP.
  • If DHCP options are not specified in the static bindings, the DHCP options from the DHCPv4 server on the segment are automatically inherited in the static bindings. However, if you have explicitly added one or more DHCP options in the static bindings, these DHCP options are not autoinherited from the DHCPv4 server on the segment.
Static Binding Options (Only in DHCPv6 Server)

The following table describes the static binding options that are available only in a DHCP for IPv6 server.

DHCP for IPv6 Option Description
DNS Servers

Optional. Enter a maximum of two domain name servers to use for the name resolution.

When not specified, no DNS is assigned to the DHCP client.
SNTP Servers

Optional. Enter a maximum of two Simple Network Time Protocol (SNTP) servers. The clients use these SNTP servers to synchronize their system time to that of the standard time servers.
Preferred Time

Optional. Enter the length of time that a valid IP address is preferred. When the preferred time expires, the IP address becomes deprecated. If no value is entered, preferred time is auto-calculated as (lease time * 0.8).

Lease time must be > preferred time.

Valid range of values is 60–4294967295. Default is 69120.
Domain Names

Optional. Enter the domain name to provide to the DHCPv6 clients. Multiple domain names are supported in an IPv6 static binding.

When not specified, no domain name is assigned to the DHCP clients.
Some additional notes for DHCPv6 static binding:
  • Gateway IP address configuration is unavailable in IPv6 static bindings. IPv6 client learns about its first-hop router from the ICMPv6 router advertisement (RA) message.
  • NTP is not supported in DHCPv6 static bindings.