diff --git a/command/agent/command.go b/command/agent/command.go
index 4abdfcc51e4..ffa1da9f279 100644
--- a/command/agent/command.go
+++ b/command/agent/command.go
@@ -589,6 +589,10 @@ func (c *Command) Run(args []string) int {
 	}
 
 	if config.Client.Enabled && config.Client.ServerJoin != nil {
+		// COMPAT: Remove in 0.10 set the default RetryInterval value, as the
+		// ServerJoin stanza is not part of a default config for an agent.
+		config.Client.ServerJoin.RetryInterval = time.Duration(30) * time.Second
+
 		joiner := retryJoiner{
 			discover:      &discover.Discover{},
 			errCh:         c.retryJoinErrCh,
diff --git a/command/agent/config.go b/command/agent/config.go
index 1bc21bb2969..07e67b52626 100644
--- a/command/agent/config.go
+++ b/command/agent/config.go
@@ -1116,6 +1116,9 @@ func (a *ServerConfig) Merge(b *ServerConfig) *ServerConfig {
 		result.EncryptKey = b.EncryptKey
 	}
 	if b.ServerJoin != nil {
+		// // COMPAT: Remove in 0.10 - ServerJoin is not defined by default on an
+		// agent config, this should be eventually moved to DefaultConfig
+		result.ServerJoin = getDefaultServerJoin()
 		result.ServerJoin = result.ServerJoin.Merge(b.ServerJoin)
 	}
 
@@ -1135,6 +1138,12 @@ func (a *ServerConfig) Merge(b *ServerConfig) *ServerConfig {
 	return &result
 }
 
+func getDefaultServerJoin() *ServerJoin {
+	return &ServerJoin{
+		RetryInterval: time.Duration(30) * time.Second,
+	}
+}
+
 // Merge is used to merge two client configs together
 func (a *ClientConfig) Merge(b *ClientConfig) *ClientConfig {
 	result := *a
@@ -1226,7 +1235,10 @@ func (a *ClientConfig) Merge(b *ClientConfig) *ClientConfig {
 	}
 
 	if b.ServerJoin != nil {
-		result.ServerJoin = b.ServerJoin
+		// // COMPAT: Remove in 0.10 - ServerJoin is not defined by default on an
+		// agent config, this should be eventually moved to DefaultConfig
+		result.ServerJoin = getDefaultServerJoin()
+		result.ServerJoin = result.ServerJoin.Merge(b.ServerJoin)
 	}
 
 	return &result
diff --git a/command/agent/retry_join.go b/command/agent/retry_join.go
index c3e9bf4754a..e49412d8c80 100644
--- a/command/agent/retry_join.go
+++ b/command/agent/retry_join.go
@@ -69,11 +69,8 @@ func (r *retryJoiner) Validate(config *Config) error {
 		if config.Server.RetryMaxAttempts != 0 {
 			return fmt.Errorf("server_join and retry_max cannot both be defined; try defining only server_join")
 		}
-		if config.Server.RetryInterval != "0" && config.Server.RetryInterval != "" {
-			// 30s is the default value that is set, ignore if this is the case
-			if config.Server.RetryInterval != "30s" {
-				return fmt.Errorf("server_join and retry_interval cannot both be defined; prefer setting the server_join parameter")
-			}
+		if config.Server.RetryInterval != "30s" {
+			return fmt.Errorf("server_join and retry_interval cannot both be defined; prefer setting the server_join parameter")
 		}
 
 		if len(config.Server.ServerJoin.RetryJoin) != 0 && len(config.Server.ServerJoin.StartJoin) != 0 {
diff --git a/command/agent/retry_join_test.go b/command/agent/retry_join_test.go
index d5479cfb06a..f1eb7fd3541 100644
--- a/command/agent/retry_join_test.go
+++ b/command/agent/retry_join_test.go
@@ -339,7 +339,7 @@ func TestRetryJoin_Validate(t *testing.T) {
 					},
 					StartJoin:        []string{},
 					RetryMaxAttempts: 0,
-					RetryInterval:    "0",
+					RetryInterval:    "30s",
 					RetryJoin:        []string{},
 				},
 			},