NSX-T DHCP Relay - Ignore the Broadcast flag set to 1 (Broadcast).

Products

VMware NSX

Issue/Introduction

Symptoms:

This use case was discovered with the usage of the Windows Feature: "Failover Cluster Manager".
Windows server is configured to receive an IP via DHCP. This is working fine.
NSX-T DHCP Relay is configured.
When configuring "Failover Cluster Manager" on the Windows Server and set the service to get an IP via DHCP it doesn't work. DHCP Request is timing out.
From taking packet capture on the Windows VM we could see:

DHCP Discover: Broadbact message: Ethernet II, Src: VMware_82:13:08 (##:##:##:##:##:08), Dst: Broadcast (##:##:##:##:##:ff)

Dynamic Host Configuration Protocol (Discover)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x14f11746
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast <----------- Broadcast flag set to 1.
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 0.0.0.0
    Client MAC address: VMware_82:13:08 (##:##:##:##:##:08)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Discover)
    Option: (116) DHCP Auto-Configuration
    Option: (61) Client identifier
    Option: (12) Host Name
    Option: (60) Vendor class identifier
    Option: (55) Parameter Request List
    Option: (255) End

Therefore the reply coming from the DHCP Relay is not a broadcast one.
DHCP Offer received on the Windows VM: Ethernet II, Src: ##:##:##:##:##:52 (##:##:##:##:##:52), Dst: VMware_82:13:08 (##:##:##:##:##:08)
Even if the flag was set to Broadcast 1:

Dynamic Host Configuration Protocol (Offer)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x14f11746
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast <----------- Broadcast flag set to 1.
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0
    Your (client) IP address: 10.38.0.248
    Next server IP address: 0.0.0.0
    Relay agent IP address: 0.0.0.0
    Client MAC address: VMware_82:13:08 (##:##:##:##:##:08)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Offer)
    Option: (54) DHCP Server Identifier (10.38.0.200)
    Option: (51) IP Address Lease Time
    Option: (1) Subnet Mask (255.255.255.0)
    Option: (3) Router
    Option: (255) End
    Padding: 000000000000000000000000000000000000000000000000…

Environment

VMware NSX-T Data Center

Cause

NSX-T DHCP Relay is ignoring the DHCP flag broadcast.

Resolution

Fix is in NSX-T 3.1.1

Workaround:
There is no workaround within the NSX-T DHCP. You can either:

Configure the Windows "Failover Cluster Manager" to use static IP instead of DHCP.

or

Bypass the NSX-T DHCP Relay.