Knowledge Base

The ESX/ESXi VMkernel's network stack does not support multi-homing or multiple routing tables.

 

The VMkernel TCP/IP stack uses a single routing table to route traffic. If you have multiple VMkernel network interfaces (vmknics) that belong to the same IP subnet, the vmkernel TCP/IP stack picks one of the interfaces for all out going traffic on that subnet as dictated by the routing table.

For example, if you have vmkernel ports configured as follows:

• One vmkernel port for vMotion, say vmk0
• Another vmkernel port for iSCSI, say vmk1

If both of these vmknics are configured to be on the same IP subnet, the vmkernel TCP/IP stack chooses one of the two interfaces for all VMkernel traffic (vMotion and NFS) going out on that subnet.

Configurations having more than one vmknic interface on the same IP subnet should be avoided, unless the vmknics on the same subnet are bound together using port-binding (such as for iSCSI).

Take special care when migrating an ESX system to ESXi. On ESX, the management interface (vswif) is owned by Service Console and any routing decision for outgoing management traffic are done using the routing table in the Service Console. VMkernel interfaces (vmknics), however, are owned by the VMkernel and any routing decision for outgoing VMkernel traffic (vMotion, FT, iSCSI and NFS) are done using the routing table in the VMkernel TCP/IP stack.

On ESX, the management interface can be in the same IP subnet as one of the VMkernel NICs without causing any issues.

ESXi does not have a Service Console. The management agents/daemons run on VMkernel and use the VMkernel's TCP/IP stack. As such, the management interface is just another VMkernel interface (vmknic). On ESXi, the management traffic and the VMkernel traffic use the same routing table in the VMkernel TCP/IP stack.

Therefore, there is a possibility of unexpected network behavior if the management interface is on the same IP subnet as one of the other VMkernel interfaces.

Note: Having more than one vmknic using the same DHCP server for configuration leads to the same situation. VMware recommends avoiding this scenario as well.