diff --git a/nova/compute/manager.py b/nova/compute/manager.py
index 9afc2641e0..ce6b98ddcd 100644
--- a/nova/compute/manager.py
+++ b/nova/compute/manager.py
@@ -3051,6 +3051,7 @@ class ComputeManager(manager.Manager):
                 self._build_resources_cleanup(instance, network_info)
         except (exception.UnexpectedTaskStateError,
                 exception.InstanceUnacceptable,
+                exception.ExternalNetworkAttachForbidden,
                 exception.OverQuota, exception.InvalidBDM) as e:
             self._build_resources_cleanup(instance, network_info)
             raise exception.BuildAbortException(instance_uuid=instance.uuid,
diff --git a/nova/tests/unit/compute/test_compute_mgr.py b/nova/tests/unit/compute/test_compute_mgr.py
index bef47424a2..4b6980aff8 100644
--- a/nova/tests/unit/compute/test_compute_mgr.py
+++ b/nova/tests/unit/compute/test_compute_mgr.py
@@ -8605,6 +8605,36 @@ class ComputeManagerBuildInstanceTestCase(test.NoDBTestCase):
             mock.ANY, network_arqs)
         return resources
 
+    @mock.patch.object(virt_driver.ComputeDriver,
+                       'prepare_for_spawn')
+    @mock.patch.object(nova.compute.manager.ComputeManager,
+                       '_build_networks_for_instance')
+    @mock.patch.object(virt_driver.ComputeDriver,
+                       'prepare_networks_before_block_device_mapping')
+    def test_build_resources_prepnets_exception(
+    self, mock_prep_net, mock_build_net, mock_prep_spawn):
+
+        args = (self.context, self.instance, self.requested_networks,
+                self.security_groups, self.image, self.block_device_mapping,
+                self.resource_provider_mapping, [])
+
+        mock_prep_net.side_effect = exception.ExternalNetworkAttachForbidden(
+            network_uuid=uuids.network_id)
+
+        resources = self.compute._build_resources(*args)
+        e = self.assertRaises(
+            exception.BuildAbortException,
+            resources.__enter__
+        )
+        self.assertIn(
+            "It is not allowed to create an interface on external network",
+            str(e)
+        )
+
+        mock_build_net.assert_called_once_with(self.context, self.instance,
+            self.requested_networks, mock.ANY,
+            mock.ANY, mock.ANY)
+
     @mock.patch.object(nova.compute.manager.ComputeManager,
                        '_get_bound_arq_resources')
     def test_accel_build_resources_no_device_profile(self, mock_get_arqs):
diff --git a/releasenotes/notes/fix-failure-prep-block-d7e1cad6d3910502.yaml b/releasenotes/notes/fix-failure-prep-block-d7e1cad6d3910502.yaml
new file mode 100644
index 0000000000..5107335c66
--- /dev/null
+++ b/releasenotes/notes/fix-failure-prep-block-d7e1cad6d3910502.yaml
@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    Fixes `bug 2137673`_ where details about the fact that the user specified
+    an incorrect configuration by requesting to attach to an external network are
+    masked by "Failure prepping block device" since they do not have access to
+    the whole backtrace.
+
+    .. _bug 2137673: https://bugs.launchpad.net/nova/+bug/2137673