Providers can support multiple box formats if they choose to.

Currently, providers must match a box format exactly the same
as that provider's name. i.e. the virtuabox provider needs a
"virtualbox" box and the "vmware_fusion" provider needs a
"vmware_fusion" box. Now, the provider can specify what the box format
is they want and support multiple if wanted.

Other box formats are specified in the provider definition within
a plugin:

    class Plugin < Vagrant.plugin("2", "provider")
      # ... other stuff

      provider("foo", box_format: ["virtualbox", "other_format"]) do
        # .. same
      end
    end

Now when using the example "foo" provider above, boxes for both
"virtualbox" or "other_format" are searched for. If both are found,
the order in which the formats exist determines precedence.

CHANGELOG.md

@@ -30,6 +30,8 @@ FEATURES:
     certain guest operating systems can implement. This allows greater
     flexibility in doing guest-specific behavior.
   - Ansible provisioner support. [GH-1465]
+  - Providers can now support multiple box formats by specifying the
+    `box_format:` option.
 
 IMPROVEMENTS:
 

lib/vagrant/environment.rb

@@ -328,13 +328,27 @@ module Vagrant
 
       box = nil
       if config.vm.box
-        begin
-          box = boxes.find(config.vm.box, provider)
-        rescue Errors::BoxUpgradeRequired
-          # Upgrade the box if we must
-          @logger.info("Upgrading box during config load: #{config.vm.box}")
-          boxes.upgrade(config.vm.box)
-          retry
+        # Determine the box formats we support
+        formats = provider_options[:box_format]
+        formats ||= provider
+        formats = [formats] if !formats.is_a?(Array)
+
+        @logger.info("Provider-supported box formats: #{formats.inspect}")
+        formats.each do |format|
+          begin
+            box = boxes.find(config.vm.box, format.to_s)
+          rescue Errors::BoxUpgradeRequired
+            # Upgrade the box if we must
+            @logger.info("Upgrading box during config load: #{config.vm.box}")
+            boxes.upgrade(config.vm.box)
+            retry
+          end
+
+          # If a box was found, we exit
+          if box
+            @logger.info("Box found with format: #{format}")
+            break
+          end
         end
       end
 

test/unit/vagrant/environment_test.rb

@@ -408,11 +408,11 @@ VF
 
   describe "getting a machine" do
     # A helper to register a provider for use in tests.
-    def register_provider(name, config_class=nil)
+    def register_provider(name, config_class=nil, options=nil)
       provider_cls = Class.new(Vagrant.plugin("2", :provider))
 
       register_plugin("2") do |p|
-        p.provider(name) { provider_cls }
+        p.provider(name, options) { provider_cls }
 
         if config_class
           p.config(name, :provider) { config_class }
@@ -572,6 +572,56 @@ VF
       machine.config.ssh.port.should == 100
     end
 
+    it "should load the box configuration for other formats for a V2 box" do
+      register_provider("foo", nil, box_format: "bar")
+
+      environment = isolated_environment do |env|
+        env.vagrantfile(<<-VF)
+Vagrant.configure("2") do |config|
+  config.vm.box = "base"
+end
+VF
+
+        env.box2("base", :bar, :vagrantfile => <<-VF)
+Vagrant.configure("2") do |config|
+  config.ssh.port = 100
+end
+VF
+      end
+
+      env = environment.create_vagrant_env
+      machine = env.machine(:default, :foo)
+      machine.config.ssh.port.should == 100
+    end
+
+    it "prefer sooner formats when multiple box formats are available" do
+      register_provider("foo", nil, box_format: ["fA", "fB"])
+
+      environment = isolated_environment do |env|
+        env.vagrantfile(<<-VF)
+Vagrant.configure("2") do |config|
+  config.vm.box = "base"
+end
+VF
+
+        env.box2("base", :fA, :vagrantfile => <<-VF)
+Vagrant.configure("2") do |config|
+  config.ssh.port = 100
+end
+VF
+
+        env.box2("base", :fB, :vagrantfile => <<-VF)
+Vagrant.configure("2") do |config|
+  config.ssh.port = 200
+end
+VF
+      end
+
+      env = environment.create_vagrant_env
+      machine = env.machine(:default, :foo)
+      machine.config.ssh.port.should == 100
+    end
+
     it "should load the provider override if set" do
       register_provider("bar")
       register_provider("foo")