Virtual Machine PXE client uses the DHCP Relay address as a default gateway instead of the default gateway provided by the DHCP server

Products

• VMware vSphere ESXi

Issue/Introduction

Symptoms:

• The PXE virtual machine obtains a DHCP address successfully but times out accessing the TFTP server during the PXE process
• The packet capture analysis shows that the virtual machine is using the DHCP Relay address for the default gateway instead of the default gateway supplied by the DHCP server
  
• The PXE virtual machine is booting in an environment that uses DHCP Relay
• The DHCP Relay interface for the subnet is not the same as the default gateway
• The DHCP server and the TFTP server are located on separate VLANs from the PXE client
  
• The virtual machine uses the E1000 or VMXNET3 virtual NIC
  

Environment

VMware vSphere ESXi 5.5
VMware vSphere ESXi 5.0
VMware vSphere ESXi 5.1

Cause

This issue occurs under these circumstances:

• PXE client subnet: X.X.X.0/24
  
• Physical network uses redundant routers: X.X.X.2 and X.X.X.3
• The default gateway is a virtual IP (VIP): X.X.X.1
• DHCP Relay operates on either X.X.X.2 or X.X.X.3 but these interfaces do not route traffic
  
• DHCP server and TFTP server used by the PXE process are on a remote VLAN

In this scenario, the virtual machine PXE BIOS uses the DHCP relay interface as the default gateway instead of the default gateway supplied by the DHCP server. The virtual machine is unable to reach the TFTP server because the DHCP relay interface does not route the packets.

Resolution

This isssue does not occur in ESXi 6.0 The PXE BIOS for VMs running on ESXi 6.0 hosts uses the default gateway supplied by the DHCP server.

To resolve this issue, upgrade to ESXi 6.0.
To work around this issue on ESXi 5.x, use one of these options.:

• Locate the DHCP server on the same VLAN as the PXE clients
• Locate the TFTP server used by the PXE process on the same VLAN as the PXE clients
  

Additional Information

虚拟机 PXE 客户端使用 DHCP 中继地址作为默认网关，而不是由 DHCP 服务器所提供的默认网关
仮想マシン PXE クライアントは、DHCP サーバにより提供されたデフォルトのゲートウェイではなく DHCP リレー アドレスをデフォルトのゲートウェイとして使用する