romiscuous mode is enabled at the portgroup and the virtual switch level

by Hojung posted Nov 22, 2012


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About port group and virtual switch

ESX servers have a whole virtual network within them: guest machines connect to virtual switches and uplinks from these switches to the outside. Although, one term that is not used outside of ESX are "port groups." After reading a bit about them and looking at the various tools the the ESX console provides, I think the best way for a network engineer to understand port groups is to see them as network hubs connected to a single vswitch port. This actually makes sense for multiple reasons:

  • All members of a port group share common attributes like a VLAN tag
  • All members of a port group can see all of the packets sent by other members of this port group
  • A port group is always connected to a single vswitch
Actually, it even makes sense to think of the VLAN tag as being applied to the vswitch port that is connected to the uplink of the virtual hub. Therefore, a vswitch with a portgroup "PG1" that has two members "VG1" and "VG2" would be built using a pswitch and a 3-port hub. The uplink of the hub is connected to a pswitch port. Applying a VLAN tag on that port group then corresponds to configuring the VLAN on the pswitch port.

Port groups in ESX are identified by their name, which must be unique within an ESX server. Having the same port group names in different ESX servers, however, makes a lot of sense, especially when moving guests around between them. More on this later.

How promiscuous mode works at the virtual switch and portgroup levels


Promiscuous mode is a security policy which can be defined at the virtual switch or portgroup level in vSphere ESX/ESXi. A virtual machine, Service Console or VMkernel network interface in a portgroup which allows use of promiscuous mode can see all network traffic traversing the virtual switch.

By default, a guest operating system's virtual network adapter only receives frames that are meant for it. Placing the guest's network adapter in promiscuous mode causes it to receive all frames passed on the virtual switch that are allowed under the VLAN policy for the associated portgroup. This can be useful for intrusion detection monitoring or if a sniffer needs to to analyze all traffic on the network segment.

For more information on configuring a virtual switch or portgroup to allow promiscuous mode, see Configuring promiscuous mode on a virtual switch or portgroup (1004099).


When promiscuous mode is enabled at the portgroup level, objects defined within that portgroup have the option of receiving all incoming traffic on the vSwitch. Interfaces and virtual machines within the portgroup will be able to see all traffic passing on the vSwitch, but all other portgroups within the same virtual switch do not.

When promiscuous mode is enabled at the virtual switch level, all portgroups within the vSwitch will default to allowing promiscuous mode. However, promiscuous mode can be explicitly disabled at one or more portgroups within the vSwitch, which override the vSwitch-defined default.

If software within a virtual machine is attempting to put the guest network adapter in promiscuous mode, contrary to the defined vSwitch or portgroup security policy, it may be necessary to investigate if the virtual machine is running undesired software. For more information, see Identifying virtual machines attempting to use promiscuous network mode on ESX/ESXi (1023341).

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