diff --git a/cmd/commands.go b/cmd/commands.go
index 3daca6ab..e58c9539 100644
--- a/cmd/commands.go
+++ b/cmd/commands.go
@@ -55,7 +55,7 @@ const (
 	explainHelp                 = "flag to explain connectivity output with rules explanations per allowed/denied connections (default false)"
 	synthesisDumpDirHelp        = "apply synthesis; specify directory path to store k8s synthesis results"
 	synthesizeAdminPoliciesHelp = "include admin network policies in policy synthesis (default false)"
-	outputFormatHelp            = "output format; must be one of [txt, dot, json, svg]"
+	outputFormatHelp            = "output format; must be one of "
 	outputFilterFlagHelp        = "filter the analysis results by vm names, can specify more than one (example: \"vm1,vm2\")"
 	quietHelp                   = "flag to run quietly, report only severe errors and result (default false)"
 	verboseHelp                 = "flag to run with more informative messages printed to log (default false)"
@@ -74,7 +74,7 @@ type inArgs struct {
 	skipAnalysis      bool
 	anonymise         bool
 	outputFile        string
-	outputFormat      string
+	outputFormat      outFormat
 	quiet             bool
 	verbose           bool
 	explain           bool
@@ -82,8 +82,12 @@ type inArgs struct {
 	color             bool
 }
 
+func newInArgs() *inArgs {
+	return &inArgs{outputFormat: outFormatText} // init with default val for outputFormat
+}
+
 func newRootCommand() *cobra.Command {
-	args := &inArgs{}
+	args := newInArgs()
 	rootCmd := &cobra.Command{
 		Use: "nsxanalyzer",
 		Short: `nsxanalyzer is a CLI for collecting NSX resources, analysis of permitted connectivity between VMs,
@@ -117,8 +121,7 @@ and generation of k8s network policies. It uses REST API calls from NSX manager.
 	rootCmd.PersistentFlags().StringVar(&args.synthesisDumpDir, synthesisDumpDirFlag, "", synthesisDumpDirHelp)
 	rootCmd.PersistentFlags().BoolVar(&args.synthesizeAdmin, synthesizeAdminPoliciesFlag, false, synthesizeAdminPoliciesHelp)
 	rootCmd.PersistentFlags().StringVarP(&args.outputFile, outputFileFlag, outputFileShortFlag, "", outputFileHelp)
-	// todo - check if the format is valid
-	rootCmd.PersistentFlags().StringVarP(&args.outputFormat, outputFormatFlag, outputFormantShortFlag, common.TextFormat, outputFormatHelp)
+	rootCmd.PersistentFlags().VarP(&args.outputFormat, outputFormatFlag, outputFormantShortFlag, outputFormatHelp+allFormatsStr)
 	rootCmd.PersistentFlags().BoolVarP(&args.quiet, quietFlag, "q", false, quietHelp)
 	rootCmd.PersistentFlags().BoolVarP(&args.verbose, verboseFlag, "v", false, verboseHelp)
 	rootCmd.PersistentFlags().BoolVarP(&args.explain, explainFlag, "e", false, explainHelp)
@@ -187,7 +190,7 @@ func runCommand(args *inArgs) error {
 		}
 	}
 	if args.topologyDumpFile != "" {
-		topology, err := resources.OutputTopologyGraph(args.topologyDumpFile, args.outputFormat)
+		topology, err := resources.OutputTopologyGraph(args.topologyDumpFile, args.outputFormat.String())
 		if err != nil {
 			return err
 		}
@@ -195,7 +198,7 @@ func runCommand(args *inArgs) error {
 	}
 	if !args.skipAnalysis {
 		params := common.OutputParameters{
-			Format:   args.outputFormat,
+			Format:   args.outputFormat.String(),
 			FileName: args.outputFile,
 			VMs:      args.outputFilter,
 			Explain:  args.explain,
diff --git a/cmd/enum_flags.go b/cmd/enum_flags.go
new file mode 100644
index 00000000..b5ef4891
--- /dev/null
+++ b/cmd/enum_flags.go
@@ -0,0 +1,40 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/np-guard/vmware-analyzer/pkg/common"
+)
+
+type outFormat string
+
+var (
+	outFormatText outFormat = common.TextFormat
+	outFormatDot  outFormat = common.DotFormat
+	outFormatSvg  outFormat = common.SvgFormat
+	outFormatJSON outFormat = common.JSONFormat
+)
+
+var allFormats = []*outFormat{&outFormatText, &outFormatDot, &outFormatSvg, &outFormatJSON}
+var allFormatsStr = common.JoinStringifiedSlice(allFormats, common.CommaSeparator)
+
+// String is used both by fmt.Print and by Cobra in help text
+func (e *outFormat) String() string {
+	return string(*e)
+}
+
+// Set must have pointer receiver so it doesn't change the value of a copy
+func (e *outFormat) Set(v string) error {
+	switch v {
+	case common.TextFormat, common.DotFormat, common.JSONFormat, common.SvgFormat:
+		*e = outFormat(v)
+		return nil
+	default:
+		return fmt.Errorf("must be one of %s", allFormatsStr)
+	}
+}
+
+// Type is only used in help text
+func (e *outFormat) Type() string {
+	return "string"
+}
diff --git a/cmd/main_test.go b/cmd/main_test.go
index 047af6e8..63e56606 100644
--- a/cmd/main_test.go
+++ b/cmd/main_test.go
@@ -61,6 +61,11 @@ const (
 )
 
 var staticTests = []*cliTest{
+	{
+		name:        "unsupported_format_check",
+		args:        "-r ../pkg/collector/data/json/Example1.json -v -o svg -o ex1.svg ",
+		expectedErr: []string{"invalid argument"},
+	},
 	{
 		// version
 		name:                    "version",
diff --git a/pkg/model/connectivity/output.go b/pkg/model/connectivity/output.go
index 4149135f..68e90514 100644
--- a/pkg/model/connectivity/output.go
+++ b/pkg/model/connectivity/output.go
@@ -1,6 +1,10 @@
 package connectivity
 
-import "github.com/np-guard/vmware-analyzer/pkg/common"
+import (
+	"fmt"
+
+	"github.com/np-guard/vmware-analyzer/pkg/common"
+)
 
 func (c ConnMap) GenTextualConnectivityOutput() (res string, err error) {
 	return c.GenConnectivityOutput(common.OutputParameters{Format: common.TextFormat})
@@ -16,6 +20,8 @@ func (c ConnMap) GenConnectivityOutput(params common.OutputParameters) (res stri
 		g = common.NewEdgesGraph(common.AnalyzedConnectivityHeader, []string{"Source", "Destination", "Permitted connections"}, params.Color)
 	case common.DotFormat, common.SvgFormat:
 		g = common.NewDotGraph(false)
+	default:
+		return "", fmt.Errorf("unsupported format %s", params.Format)
 	}
 	for _, e := range filteredConn.toSlice() {
 		if !e.DetailedConn.Conn.IsEmpty() {