The MIB module for creation and deletion of Virtual Interfaces
and Virtual Interface Groups.  In addition to this MIB,
interface mib (rfc2863) will also contain entries for virtual
interfaces.

There are two ways virtual interfaces can be created and bound
to an interface. The MIB Object cvifGroupsSupported indicates
if a particular system supports the Virtual Interface Group
concept.

If Virtual Interface Groups are supported, then a virtual
interface is always created/deleted in conjunction with its
associated Virtual Interface Group. The Virtual Interface
Group, which may be associated to multiple virtual interfaces,
is then bound to a physical interface or another virtual
interface like an EtherChannel.

If Virtual Interface Groups are not supported, then a virtual
interface is created/deleted independently. And each virtual
interface is bound to a physical interface or another virtual
interface like an EtherChannel.

In order for a virtual interface to become operational, it must
be directly or indirectly (via its Virtual Interface Group)
bound to a physical interface or possibly to another virtual
interface such as an EtherChannel.

The cvifGroupTable is only used if the MIB object
cvifGroupsSupported indicates Virtual Interface Groups are
supported.