diff --git a/web/federate.go b/web/federate.go
index 2423b2577..1c5771bdf 100644
--- a/web/federate.go
+++ b/web/federate.go
@@ -74,6 +74,16 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 	}
 	sort.Sort(byName(vector))
 
+	externalLabels := h.externalLabels.Clone()
+	if _, ok := externalLabels[model.InstanceLabel]; !ok {
+		externalLabels[model.InstanceLabel] = ""
+	}
+	externalLabelNames := make(model.LabelNames, 0, len(externalLabels))
+	for ln := range externalLabels {
+		externalLabelNames = append(externalLabelNames, ln)
+	}
+	sort.Sort(externalLabelNames)
+
 	var (
 		lastMetricName model.LabelValue
 		protMetricFam  *dto.MetricFamily
@@ -86,14 +96,13 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 		}
 
 		// Sort labelnames for unittest consistency.
-		labelnames := make([]string, 0, len(s.Metric))
+		labelNames := make(model.LabelNames, 0, len(s.Metric))
 		for ln := range s.Metric {
-			labelnames = append(labelnames, string(ln))
+			labelNames = append(labelNames, ln)
 		}
-		sort.Strings(labelnames)
+		sort.Sort(labelNames)
 
-		for _, labelname := range labelnames {
-			ln := model.LabelName(labelname)
+		for _, ln := range labelNames {
 			lv := s.Metric[ln]
 			if lv == "" {
 				// No value means unset. Never consider those labels.
@@ -127,7 +136,7 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 				Name:  proto.String(string(ln)),
 				Value: proto.String(string(lv)),
 			})
-			if _, ok := h.externalLabels[ln]; ok {
+			if _, ok := externalLabels[ln]; ok {
 				globalUsed[ln] = struct{}{}
 			}
 		}
@@ -136,7 +145,8 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 			continue
 		}
 		// Attach global labels if they do not exist yet.
-		for ln, lv := range h.externalLabels {
+		for _, ln := range externalLabelNames {
+			lv := externalLabels[ln]
 			if _, ok := globalUsed[ln]; !ok {
 				protMetric.Label = append(protMetric.Label, &dto.LabelPair{
 					Name:  proto.String(string(ln)),
diff --git a/web/federate_test.go b/web/federate_test.go
index 6539e17c3..0b0204309 100644
--- a/web/federate_test.go
+++ b/web/federate_test.go
@@ -59,72 +59,72 @@ var scenarios = map[string]struct {
 		params: "match[]=test_metric1",
 		code:   200,
 		body: `# TYPE test_metric1 untyped
-test_metric1{foo="bar"} 10000 6000000
-test_metric1{foo="boo"} 1 6000000
+test_metric1{foo="bar",instance="i"} 10000 6000000
+test_metric1{foo="boo",instance="i"} 1 6000000
 `,
 	},
 	"test_metric2": {
 		params: "match[]=test_metric2",
 		code:   200,
 		body: `# TYPE test_metric2 untyped
-test_metric2{foo="boo"} 1 6000000
+test_metric2{foo="boo",instance="i"} 1 6000000
 `,
 	},
 	"test_metric_without_labels": {
 		params: "match[]=test_metric_without_labels",
 		code:   200,
 		body: `# TYPE test_metric_without_labels untyped
-test_metric_without_labels 1001 6000000
+test_metric_without_labels{instance=""} 1001 6000000
 `,
 	},
 	"{foo='boo'}": {
 		params: "match[]={foo='boo'}",
 		code:   200,
 		body: `# TYPE test_metric1 untyped
-test_metric1{foo="boo"} 1 6000000
+test_metric1{foo="boo",instance="i"} 1 6000000
 # TYPE test_metric2 untyped
-test_metric2{foo="boo"} 1 6000000
+test_metric2{foo="boo",instance="i"} 1 6000000
 `,
 	},
 	"two matchers": {
 		params: "match[]=test_metric1&match[]=test_metric2",
 		code:   200,
 		body: `# TYPE test_metric1 untyped
-test_metric1{foo="bar"} 10000 6000000
-test_metric1{foo="boo"} 1 6000000
+test_metric1{foo="bar",instance="i"} 10000 6000000
+test_metric1{foo="boo",instance="i"} 1 6000000
 # TYPE test_metric2 untyped
-test_metric2{foo="boo"} 1 6000000
+test_metric2{foo="boo",instance="i"} 1 6000000
 `,
 	},
 	"everything": {
 		params: "match[]={__name__=~'.%2b'}", // '%2b' is an URL-encoded '+'.
 		code:   200,
 		body: `# TYPE test_metric1 untyped
-test_metric1{foo="bar"} 10000 6000000
-test_metric1{foo="boo"} 1 6000000
+test_metric1{foo="bar",instance="i"} 10000 6000000
+test_metric1{foo="boo",instance="i"} 1 6000000
 # TYPE test_metric2 untyped
-test_metric2{foo="boo"} 1 6000000
+test_metric2{foo="boo",instance="i"} 1 6000000
 # TYPE test_metric_without_labels untyped
-test_metric_without_labels 1001 6000000
+test_metric_without_labels{instance=""} 1001 6000000
 `,
 	},
 	"empty label value matches everything that doesn't have that label": {
 		params: "match[]={foo='',__name__=~'.%2b'}",
 		code:   200,
 		body: `# TYPE test_metric_without_labels untyped
-test_metric_without_labels 1001 6000000
+test_metric_without_labels{instance=""} 1001 6000000
 `,
 	},
 	"empty label value for a label that doesn't exist at all, matches everything": {
 		params: "match[]={bar='',__name__=~'.%2b'}",
 		code:   200,
 		body: `# TYPE test_metric1 untyped
-test_metric1{foo="bar"} 10000 6000000
-test_metric1{foo="boo"} 1 6000000
+test_metric1{foo="bar",instance="i"} 10000 6000000
+test_metric1{foo="boo",instance="i"} 1 6000000
 # TYPE test_metric2 untyped
-test_metric2{foo="boo"} 1 6000000
+test_metric2{foo="boo",instance="i"} 1 6000000
 # TYPE test_metric_without_labels untyped
-test_metric_without_labels 1001 6000000
+test_metric_without_labels{instance=""} 1001 6000000
 `,
 	},
 	"external labels are added if not already present": {
@@ -132,12 +132,27 @@ test_metric_without_labels 1001 6000000
 		externalLabels: model.LabelSet{"zone": "ie", "foo": "baz"},
 		code:           200,
 		body: `# TYPE test_metric1 untyped
-test_metric1{foo="bar",zone="ie"} 10000 6000000
-test_metric1{foo="boo",zone="ie"} 1 6000000
+test_metric1{foo="bar",instance="i",zone="ie"} 10000 6000000
+test_metric1{foo="boo",instance="i",zone="ie"} 1 6000000
 # TYPE test_metric2 untyped
-test_metric2{foo="boo",zone="ie"} 1 6000000
+test_metric2{foo="boo",instance="i",zone="ie"} 1 6000000
 # TYPE test_metric_without_labels untyped
-test_metric_without_labels{zone="ie",foo="baz"} 1001 6000000
+test_metric_without_labels{foo="baz",instance="",zone="ie"} 1001 6000000
+`,
+	},
+	"instance is an external label": {
+		// This makes no sense as a configuration, but we should
+		// know what it does anyway.
+		params:         "match[]={__name__=~'.%2b'}", // '%2b' is an URL-encoded '+'.
+		externalLabels: model.LabelSet{"instance": "baz"},
+		code:           200,
+		body: `# TYPE test_metric1 untyped
+test_metric1{foo="bar",instance="i"} 10000 6000000
+test_metric1{foo="boo",instance="i"} 1 6000000
+# TYPE test_metric2 untyped
+test_metric2{foo="boo",instance="i"} 1 6000000
+# TYPE test_metric_without_labels untyped
+test_metric_without_labels{instance="baz"} 1001 6000000
 `,
 	},
 }
@@ -145,9 +160,9 @@ test_metric_without_labels{zone="ie",foo="baz"} 1001 6000000
 func TestFederation(t *testing.T) {
 	suite, err := promql.NewTest(t, `
 		load 1m
-			test_metric1{foo="bar"}    0+100x100
-			test_metric1{foo="boo"}    1+0x100
-			test_metric2{foo="boo"}    1+0x100
+			test_metric1{foo="bar",instance="i"}    0+100x100
+			test_metric1{foo="boo",instance="i"}    1+0x100
+			test_metric2{foo="boo",instance="i"}    1+0x100
 			test_metric_without_labels 1+10x100
 	`)
 	if err != nil {
@@ -189,7 +204,7 @@ func TestFederation(t *testing.T) {
 			t.Errorf("Scenario %q: got code %d, want %d", name, got, want)
 		}
 		if got, want := normalizeBody(res.Body), scenario.body; got != want {
-			t.Errorf("Scenario %q: got body %q, want %q", name, got, want)
+			t.Errorf("Scenario %q: got body %s, want %s", name, got, want)
 		}
 	}
 }