[client] Add support for retain existing AllowedIPs when no alternative paths are available

client/internal/routemanager/client/client.go

@@ -336,6 +336,11 @@ func (w *Watcher) recalculateRoutes(rsn reason, routerPeerStatuses map[route.ID]
 			return nil
 		}
 
+		if isRouteStickyOnFailure() {
+			log.Warnf("No available routes for network [%v], keep current route %s", w.handler, w.currentChosen.Peer)
+			return nil
+		}
+
 		if err := w.removeAllowedIPs(w.currentChosen, rsn); err != nil {
 			return fmt.Errorf("remove obsolete: %w", err)
 		}

client/internal/routemanager/client/env.go

@@ -0,0 +1,24 @@
+package client
+
+import (
+	"os"
+	"strings"
+
+	log "github.com/sirupsen/logrus"
+)
+
+const (
+	// envRouteStickyOnFailure is used to configure if routes should be kept on failure
+	envRouteStickyOnFailure = "NB_ROUTE_STICKY_ON_FAILURE"
+)
+
+// isRouteStickyOnFailure checks if routes should be kept on failure
+func isRouteStickyOnFailure() bool {
+	stickyOnFailureEnv := os.Getenv(envRouteStickyOnFailure)
+	if stickyOnFailureEnv == "" {
+		return false
+	}
+
+	log.Infof("routes will be kept on failure as %s is set to %s", envRouteStickyOnFailure, stickyOnFailureEnv)
+	return strings.ToLower(stickyOnFailureEnv) == "true"
+}