diff --git a/src/shrpx_client_handler.cc b/src/shrpx_client_handler.cc
index 53a5a5b1..cdb20146 100644
--- a/src/shrpx_client_handler.cc
+++ b/src/shrpx_client_handler.cc
@@ -952,10 +952,12 @@ ClientHandler::get_downstream_connection(int &err, Downstream *downstream,
   err = 0;
 
   switch (faddr_->alt_mode) {
-  case ALTMODE_API:
+  case UpstreamAltMode::API:
     return std::make_unique<APIDownstreamConnection>(worker_);
-  case ALTMODE_HEALTHMON:
+  case UpstreamAltMode::HEALTHMON:
     return std::make_unique<HealthMonitorDownstreamConnection>();
+  default:
+    break;
   }
 
   auto &balloc = downstream->get_block_allocator();
diff --git a/src/shrpx_config.cc b/src/shrpx_config.cc
index e7a632c4..5e0642d9 100644
--- a/src/shrpx_config.cc
+++ b/src/shrpx_config.cc
@@ -756,7 +756,7 @@ int parse_memcached_connection_params(MemcachedConnectionParams &out,
 } // namespace
 
 struct UpstreamParams {
-  int alt_mode;
+  UpstreamAltMode alt_mode;
   bool tls;
   bool sni_fwd;
   bool proxyproto;
@@ -779,17 +779,19 @@ int parse_upstream_params(UpstreamParams &out, const StringRef &src_params) {
     } else if (util::strieq_l("no-tls", param)) {
       out.tls = false;
     } else if (util::strieq_l("api", param)) {
-      if (out.alt_mode && out.alt_mode != ALTMODE_API) {
+      if (out.alt_mode != UpstreamAltMode::NONE &&
+          out.alt_mode != UpstreamAltMode::API) {
         LOG(ERROR) << "frontend: api and healthmon are mutually exclusive";
         return -1;
       }
-      out.alt_mode = ALTMODE_API;
+      out.alt_mode = UpstreamAltMode::API;
     } else if (util::strieq_l("healthmon", param)) {
-      if (out.alt_mode && out.alt_mode != ALTMODE_HEALTHMON) {
+      if (out.alt_mode != UpstreamAltMode::NONE &&
+          out.alt_mode != UpstreamAltMode::HEALTHMON) {
         LOG(ERROR) << "frontend: api and healthmon are mutually exclusive";
         return -1;
       }
-      out.alt_mode = ALTMODE_HEALTHMON;
+      out.alt_mode = UpstreamAltMode::HEALTHMON;
     } else if (util::strieq_l("proxyproto", param)) {
       out.proxyproto = true;
     } else if (!param.empty()) {
@@ -2572,7 +2574,7 @@ int parse_config(Config *config, int optid, const StringRef &opt,
     addr.alt_mode = params.alt_mode;
     addr.accept_proxy_protocol = params.proxyproto;
 
-    if (addr.alt_mode == ALTMODE_API) {
+    if (addr.alt_mode == UpstreamAltMode::API) {
       apiconf.enabled = true;
     }
 
diff --git a/src/shrpx_config.h b/src/shrpx_config.h
index bb6fc262..8ea9c55d 100644
--- a/src/shrpx_config.h
+++ b/src/shrpx_config.h
@@ -420,13 +420,13 @@ struct AltSvc {
   uint16_t port;
 };
 
-enum UpstreamAltMode {
+enum class UpstreamAltMode {
   // No alternative mode
-  ALTMODE_NONE,
+  NONE,
   // API processing mode
-  ALTMODE_API,
+  API,
   // Health monitor mode
-  ALTMODE_HEALTHMON,
+  HEALTHMON,
 };
 
 struct UpstreamAddr {
@@ -444,7 +444,7 @@ struct UpstreamAddr {
   // domain socket, this is 0.
   int family;
   // Alternate mode
-  int alt_mode;
+  UpstreamAltMode alt_mode;
   // true if |host| contains UNIX domain socket path.
   bool host_unix;
   // true if TLS is enabled.
diff --git a/src/shrpx_connection_handler.cc b/src/shrpx_connection_handler.cc
index fa32f7fc..14c11ce4 100644
--- a/src/shrpx_connection_handler.cc
+++ b/src/shrpx_connection_handler.cc
@@ -407,7 +407,7 @@ int ConnectionHandler::handle_connection(int fd, sockaddr *addr, int addrlen,
 
   Worker *worker;
 
-  if (faddr->alt_mode == ALTMODE_API) {
+  if (faddr->alt_mode == UpstreamAltMode::API) {
     worker = workers_[0].get();
 
     if (LOG_ENABLED(INFO)) {
diff --git a/src/shrpx_downstream.cc b/src/shrpx_downstream.cc
index 2538aa95..0b20d173 100644
--- a/src/shrpx_downstream.cc
+++ b/src/shrpx_downstream.cc
@@ -610,7 +610,7 @@ bool Downstream::request_buf_full() {
   auto worker = handler->get_worker();
 
   // We don't check buffer size here for API endpoint.
-  if (faddr->alt_mode == ALTMODE_API) {
+  if (faddr->alt_mode == UpstreamAltMode::API) {
     return false;
   }
 
diff --git a/src/shrpx_http2_upstream.cc b/src/shrpx_http2_upstream.cc
index 591a6686..62999efd 100644
--- a/src/shrpx_http2_upstream.cc
+++ b/src/shrpx_http2_upstream.cc
@@ -358,8 +358,8 @@ int Http2Upstream::on_request_headers(Downstream *downstream,
   auto faddr = handler_->get_upstream_addr();
 
   // For HTTP/2 proxy, we require :authority.
-  if (method_token != HTTP_CONNECT && config->http2_proxy && !faddr->alt_mode &&
-      !authority) {
+  if (method_token != HTTP_CONNECT && config->http2_proxy &&
+      faddr->alt_mode == UpstreamAltMode::NONE && !authority) {
     rst_stream(downstream, NGHTTP2_PROTOCOL_ERROR);
     return 0;
   }
@@ -383,7 +383,8 @@ int Http2Upstream::on_request_headers(Downstream *downstream,
     if (method_token == HTTP_OPTIONS &&
         path->value == StringRef::from_lit("*")) {
       // Server-wide OPTIONS request.  Path is empty.
-    } else if (config->http2_proxy && !faddr->alt_mode) {
+    } else if (config->http2_proxy &&
+               faddr->alt_mode == UpstreamAltMode::NONE) {
       req.path = path->value;
     } else {
       req.path = http2::rewrite_clean_path(downstream->get_block_allocator(),
@@ -1026,10 +1027,11 @@ Http2Upstream::Http2Upstream(ClientHandler *handler)
 
   auto faddr = handler_->get_upstream_addr();
 
-  rv = nghttp2_session_server_new2(
-      &session_, http2conf.upstream.callbacks, this,
-      faddr->alt_mode ? http2conf.upstream.alt_mode_option
-                      : http2conf.upstream.option);
+  rv =
+      nghttp2_session_server_new2(&session_, http2conf.upstream.callbacks, this,
+                                  faddr->alt_mode != UpstreamAltMode::NONE
+                                      ? http2conf.upstream.alt_mode_option
+                                      : http2conf.upstream.option);
 
   assert(rv == 0);
 
@@ -1043,7 +1045,7 @@ Http2Upstream::Http2Upstream(ClientHandler *handler)
   entry[0].value = http2conf.upstream.max_concurrent_streams;
 
   entry[1].settings_id = NGHTTP2_SETTINGS_INITIAL_WINDOW_SIZE;
-  if (faddr->alt_mode) {
+  if (faddr->alt_mode != UpstreamAltMode::NONE) {
     entry[1].value = (1u << 31) - 1;
   } else {
     entry[1].value = http2conf.upstream.window_size;
@@ -1070,7 +1072,7 @@ Http2Upstream::Http2Upstream(ClientHandler *handler)
   }
 
   auto window_size =
-      faddr->alt_mode
+      faddr->alt_mode != UpstreamAltMode::NONE
           ? std::numeric_limits<int32_t>::max()
           : http2conf.upstream.optimize_window_size
                 ? std::min(http2conf.upstream.connection_window_size,
@@ -1186,7 +1188,7 @@ int Http2Upstream::on_write() {
 
       if (http2conf.upstream.optimize_window_size) {
         auto faddr = handler_->get_upstream_addr();
-        if (!faddr->alt_mode) {
+        if (faddr->alt_mode == UpstreamAltMode::NONE) {
           auto window_size = std::min(http2conf.upstream.connection_window_size,
                                       static_cast<int32_t>(hint.rwin * 2));
 
@@ -1980,7 +1982,7 @@ int Http2Upstream::consume(int32_t stream_id, size_t len) {
 
   auto faddr = handler_->get_upstream_addr();
 
-  if (faddr->alt_mode) {
+  if (faddr->alt_mode != UpstreamAltMode::NONE) {
     return 0;
   }
 
diff --git a/src/shrpx_https_upstream.cc b/src/shrpx_https_upstream.cc
index 0e34e197..3e5a5d94 100644
--- a/src/shrpx_https_upstream.cc
+++ b/src/shrpx_https_upstream.cc
@@ -413,7 +413,8 @@ int htp_hdrs_completecb(http_parser *htp) {
 
   // mruby hook may change method value
 
-  if (req.no_authority && config->http2_proxy && !faddr->alt_mode) {
+  if (req.no_authority && config->http2_proxy &&
+      faddr->alt_mode == UpstreamAltMode::NONE) {
     // Request URI should be absolute-form for client proxy mode
     return -1;
   }
@@ -464,7 +465,7 @@ int htp_hdrs_completecb(http_parser *htp) {
     return -1;
   }
 
-  if (faddr->alt_mode) {
+  if (faddr->alt_mode != UpstreamAltMode::NONE) {
     // Normally, we forward expect: 100-continue to backend server,
     // and let them decide whether responds with 100 Continue or not.
     // For alternative mode, we have no backend, so just send 100