diff --git a/ansible/roles/prometheus/defaults/main.yml b/ansible/roles/prometheus/defaults/main.yml
index 70e5a6f509..e1fcecce7a 100644
--- a/ansible/roles/prometheus/defaults/main.yml
+++ b/ansible/roles/prometheus/defaults/main.yml
@@ -65,6 +65,7 @@ prometheus_services:
         port: "{{ prometheus_alertmanager_port }}"
         auth_user: "{{ prometheus_alertmanager_user }}"
         auth_pass: "{{ prometheus_alertmanager_password }}"
+        active_passive: "{{ prometheus_alertmanager_active_passive | bool }}"
       prometheus_alertmanager_external:
         enabled: "{{ enable_prometheus_alertmanager_external | bool }}"
         mode: "http"
@@ -72,6 +73,7 @@ prometheus_services:
         port: "{{ prometheus_alertmanager_port }}"
         auth_user: "{{ prometheus_alertmanager_user }}"
         auth_pass: "{{ prometheus_alertmanager_password }}"
+        active_passive: "{{ prometheus_alertmanager_active_passive | bool }}"
   prometheus-openstack-exporter:
     container_name: "prometheus_openstack_exporter"
     group: "prometheus-openstack-exporter"
@@ -132,6 +134,7 @@ prometheus_mysql_exporter_database_user: "{% if use_preconfigured_databases | bo
 # HAProxy
 ####################
 prometheus_active_passive: true
+prometheus_alertmanager_active_passive: true
 
 ####################
 # Blackbox
diff --git a/releasenotes/notes/prometheus-alertmanager-active-passive-74cd994cdc893494.yaml b/releasenotes/notes/prometheus-alertmanager-active-passive-74cd994cdc893494.yaml
new file mode 100644
index 0000000000..f886c70cae
--- /dev/null
+++ b/releasenotes/notes/prometheus-alertmanager-active-passive-74cd994cdc893494.yaml
@@ -0,0 +1,10 @@
+---
+upgrade:
+  - |
+    Prometheus Alertmanager has been switched to active/passive mode. This is
+    enabled by default but can be turned off by setting
+    ``prometheus_alertmanager_active_passive`` to ``no``.
+fixes:
+  - |
+    Fixes an issue with Alertmanager silence creation leading to a 404 page.
+    `LP#1987866 <https://launchpad.net/bugs/1987866>`__