diff --git a/README.md b/README.md
index 94eb860b..677f5bdf 100644
--- a/README.md
+++ b/README.md
@@ -321,6 +321,8 @@ amtool config routes --alertmanager.url=http://localhost:9090
 
 AlertManager's high availability is in production use at many companies.
 
+> Important: Both UDP and TCP are needed in alertmanager 0.15 and higher for the cluster to work.
+
 To create a highly available cluster of the Alertmanager the instances need to
 be configured to communicate with each other. This is configured using the
 `--cluster.*` flags.