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

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Article ID: 330471

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Updated On:

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.