Unexpected or extraneous IP address usage by virtual machines (VMs)

Products

• VMware vSphere ESXi

Issue/Introduction

It may be observed in network monitoring tools, IP address management (IPAM) systems, DHCP servers and similar tools that virtual machines (VMs) are associated with more than one IP address. The pool of available IP addresses on a DHCP server may have higher than expected utilization and a single MAC address (media access control address) may be associated with more than one DHCP lease.

Cause

If a network boot option such as PXE or BOOTP is enabled on a virtual machine, the network boot client embedded in the BIOS/UEFI will search for (discover) a DHCP server and may subsequently receive an IP address offer. This client can run prior to other boot options, and an IP offer can be sent, whether or not the virtual machine ultimately boots from the network. 

After the virtual machine's operating system (OS) starts, a separate DHCP client provided by the OS vendor will initialize and will make a second DHCP request (discover) from the same MAC address, and may receive another IP address, if the network interface in the VM is configured for dynamic addressing.

Depending on your DHCP server's behavior and configuration options, an existing IP lease - such as the IP address first received during a network boot attempt - may or may not be reused for a subsequent DHCP IP offer.

Resolution

The broadcast of two DHCP discover messages during the boot process is expected during the startup of a VM or network device that has the network boot feature turned on.

To avoid excessive utilization of IP addresses, your DHCP server may offer configuration options that: 

• allow you to assign a reserved IP address to a MAC address
• limit clients to a single active DHCP IP address lease, returning the same IP for subsequent requests

Alternatively, the duration of IP leases can be reduced or network boot can be disabled on VMs if appropriate.

Contact your DHCP server vendor for details on possible configuration options.