diff --git a/lib/spack/spack/solver/asp.py b/lib/spack/spack/solver/asp.py
index 9e1e86bfe49874..22d6994e4c071f 100644
--- a/lib/spack/spack/solver/asp.py
+++ b/lib/spack/spack/solver/asp.py
@@ -1610,6 +1610,7 @@ def external_imposition(input_spec, requirements):
                 self.gen.newline()
 
             self.trigger_rules()
+            self.effect_rules()
 
     def preferred_variants(self, pkg_name):
         """Facts on concretization preferences, as read from packages.yaml"""
diff --git a/lib/spack/spack/solver/concretize.lp b/lib/spack/spack/solver/concretize.lp
index 022c6abe7803f7..70b3645810d458 100644
--- a/lib/spack/spack/solver/concretize.lp
+++ b/lib/spack/spack/solver/concretize.lp
@@ -993,14 +993,13 @@ pkg_fact(Package, variant_single_value("dev_path"))
 % Platform semantics
 %-----------------------------------------------------------------------------
 
-% if no platform is set, fall back to the default
-error(100, "platform '{0}' is not allowed on the current host", Platform)
-  :- attr("node_platform", _, Platform), not allowed_platform(Platform).
+% NOTE: Currently we have a single allowed platform per DAG, therefore there is no
+% need to have additional optimization criteria. If we ever add cross-platform dags,
+% this needs to be changed.
+:- 2 { allowed_platform(Platform) }, internal_error("More than one allowed platform detected").
 
-attr("node_platform", PackageNode, Platform)
- :- attr("node", PackageNode),
-    not attr("node_platform_set", PackageNode),
-    node_platform_default(Platform).
+1 { attr("node_platform", PackageNode, Platform) : allowed_platform(Platform) } 1
+  :- attr("node", PackageNode).
 
 % setting platform on a node is a hard constraint
 attr("node_platform", PackageNode, Platform)
diff --git a/lib/spack/spack/test/concretize.py b/lib/spack/spack/test/concretize.py
index f897230088a96a..db675231505c67 100644
--- a/lib/spack/spack/test/concretize.py
+++ b/lib/spack/spack/test/concretize.py
@@ -2398,6 +2398,19 @@ def test_reuse_specs_from_non_available_compilers(self, mutable_config, mutable_
         for s in root.traverse(root=False):
             assert s.satisfies("%gcc@10.2.1")
 
+    @pytest.mark.regression("43406")
+    def test_externals_with_platform_explicitly_set(self, tmp_path):
+        """Tests that users can specify platform=xxx in an external spec"""
+        external_conf = {
+            "mpich": {
+                "buildable": False,
+                "externals": [{"spec": "mpich@=2.0.0 platform=test", "prefix": str(tmp_path)}],
+            }
+        }
+        spack.config.set("packages", external_conf)
+        s = Spec("mpich").concretized()
+        assert s.external
+
 
 @pytest.fixture()
 def duplicates_test_repository():