diff --git a/bgpd/bgp_attr.c b/bgpd/bgp_attr.c
index 7c94f7e..829b017 100644
--- a/bgpd/bgp_attr.c
+++ b/bgpd/bgp_attr.c
@@ -62,6 +62,17 @@
   { BGP_ATTR_AS_PATHLIMIT,     "AS_PATHLIMIT" },
 };
 static const int attr_str_max = sizeof(attr_str)/sizeof(attr_str[0]);
+
+static const struct message attr_flag_str[] =
+{
+  { BGP_ATTR_FLAG_OPTIONAL, "Optional" },
+  { BGP_ATTR_FLAG_TRANS,    "Transitive" },
+  { BGP_ATTR_FLAG_PARTIAL,  "Partial" },
+  /* bgp_attr_flags_diagnose() relies on this bit being last in this list */
+  { BGP_ATTR_FLAG_EXTLEN,   "Extended Length" },
+};
+static const size_t attr_flag_str_max =
+  sizeof (attr_flag_str) / sizeof (attr_flag_str[0]);
 
 static struct hash *cluster_hash;
 
@@ -697,6 +708,40 @@
     }
 }
 
+/* Find out what is wrong with the path attribute flag bits and log the error.
+   "Flag bits" here stand for Optional, Transitive and Partial, but not for
+   Extended Length. Checking O/T/P bits at once implies, that the attribute
+   being diagnosed is defined by RFC as either a "well-known" or an "optional,
+   non-transitive" attribute. */
+static void
+bgp_attr_flags_diagnose
+(
+  struct peer * peer,
+  const u_int8_t attr_code,
+  u_int8_t desired_flags, /* how RFC says it must be */
+  u_int8_t real_flags     /* on wire                 */
+)
+{
+  u_char seen = 0, i;
+
+  desired_flags &= ~BGP_ATTR_FLAG_EXTLEN;
+  real_flags &= ~BGP_ATTR_FLAG_EXTLEN;
+  for (i = 0; i <= 2; i++) /* O,T,P, but not E */
+    if
+    (
+      CHECK_FLAG (desired_flags, attr_flag_str[i].key) !=
+      CHECK_FLAG (real_flags,    attr_flag_str[i].key)
+    )
+    {
+      zlog (peer->log, LOG_ERR, "%s attribute must%s be flagged as \"%s\"",
+            LOOKUP (attr_str, attr_code),
+            CHECK_FLAG (desired_flags, attr_flag_str[i].key) ? "" : " not",
+            attr_flag_str[i].str);
+      seen = 1;
+    }
+  assert (seen);
+}
+
 /* Get origin attribute of the update message. */
 static int
 bgp_attr_origin (struct peer *peer, bgp_size_t length, 
@@ -714,12 +759,7 @@
      attribute (type, length and value). */
   if ((flag & ~BGP_ATTR_FLAG_EXTLEN) != BGP_ATTR_FLAG_TRANS)
   {
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_OPTIONAL))
-      zlog (peer->log, LOG_ERR, "ORIGIN attribute must not be flagged as \"optional\" (%u)", flag);
-    if (! CHECK_FLAG (flag, BGP_ATTR_FLAG_TRANS))
-      zlog (peer->log, LOG_ERR, "ORIGIN attribute must be flagged as \"transitive\" (%u)", flag);
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_PARTIAL))
-      zlog (peer->log, LOG_ERR, "ORIGIN attribute must not be flagged as \"partial\" (%u)", flag);
+    bgp_attr_flags_diagnose (peer, BGP_ATTR_ORIGIN, BGP_ATTR_FLAG_TRANS, flag);
     bgp_notify_send_with_data (peer, BGP_NOTIFY_UPDATE_ERR, BGP_NOTIFY_UPDATE_ATTR_FLAG_ERR, startp, total);
     return -1;
   }
@@ -917,12 +957,7 @@
   /* Flags check. */
   if ((flag & ~BGP_ATTR_FLAG_EXTLEN) != BGP_ATTR_FLAG_TRANS)
   {
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_OPTIONAL))
-      zlog (peer->log, LOG_ERR, "NEXT_HOP attribute must not be flagged as \"optional\" (%u)", flag);
-    if (! CHECK_FLAG (flag, BGP_ATTR_FLAG_TRANS))
-      zlog (peer->log, LOG_ERR, "NEXT_HOP attribute must be flagged as \"transitive\" (%u)", flag);
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_PARTIAL))
-      zlog (peer->log, LOG_ERR, "NEXT_HOP attribute must not be flagged as \"partial\" (%u)", flag);
+    bgp_attr_flags_diagnose (peer, BGP_ATTR_NEXT_HOP, BGP_ATTR_FLAG_TRANS, flag);
     bgp_notify_send_with_data (peer, BGP_NOTIFY_UPDATE_ERR, BGP_NOTIFY_UPDATE_ATTR_FLAG_ERR, startp, total);
     return -1;
   }
@@ -977,12 +1012,7 @@
   /* Flag checks. */
   if ((flag & ~BGP_ATTR_FLAG_EXTLEN) != BGP_ATTR_FLAG_OPTIONAL)
   {
-    if (! CHECK_FLAG (flag, BGP_ATTR_FLAG_OPTIONAL))
-      zlog (peer->log, LOG_ERR, "MULTI_EXIT_DISC attribute must be flagged as \"optional\" (%u)", flag);
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_TRANS))
-      zlog (peer->log, LOG_ERR, "MULTI_EXIT_DISC attribute must not be flagged as \"transitive\" (%u)", flag);
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_PARTIAL))
-      zlog (peer->log, LOG_ERR, "MULTI_EXIT_DISC attribute must not be flagged as \"partial\" (%u)", flag);
+    bgp_attr_flags_diagnose (peer, BGP_ATTR_MULTI_EXIT_DISC, BGP_ATTR_FLAG_OPTIONAL, flag);
     bgp_notify_send_with_data (peer, BGP_NOTIFY_UPDATE_ERR, BGP_NOTIFY_UPDATE_ATTR_FLAG_ERR, startp, total);
     return -1;
   }
@@ -1018,12 +1048,7 @@
   /* Flag checks. */
   if ((flag & ~BGP_ATTR_FLAG_EXTLEN) != BGP_ATTR_FLAG_TRANS)
   {
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_OPTIONAL))
-      zlog (peer->log, LOG_ERR, "LOCAL_PREF attribute must not be flagged as \"optional\" (%u)", flag);
-    if (! CHECK_FLAG (flag, BGP_ATTR_FLAG_TRANS))
-      zlog (peer->log, LOG_ERR, "LOCAL_PREF attribute must be flagged as \"transitive\" (%u)", flag);
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_PARTIAL))
-      zlog (peer->log, LOG_ERR, "LOCAL_PREF attribute must not be flagged as \"partial\" (%u)", flag);
+    bgp_attr_flags_diagnose (peer, BGP_ATTR_LOCAL_PREF, BGP_ATTR_FLAG_TRANS, flag);
     bgp_notify_send_with_data (peer, BGP_NOTIFY_UPDATE_ERR, BGP_NOTIFY_UPDATE_ATTR_FLAG_ERR, startp, total);
     return -1;
   }
@@ -1066,12 +1091,7 @@
   /* Flag checks. */
   if ((flag & ~BGP_ATTR_FLAG_EXTLEN) != BGP_ATTR_FLAG_TRANS)
   {
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_OPTIONAL))
-      zlog (peer->log, LOG_ERR, "ATOMIC_AGGREGATE attribute must not be flagged as \"optional\" (%u)", flag);
-    if (! CHECK_FLAG (flag, BGP_ATTR_FLAG_TRANS))
-      zlog (peer->log, LOG_ERR, "ATOMIC_AGGREGATE attribute must be flagged as \"transitive\" (%u)", flag);
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_PARTIAL))
-      zlog (peer->log, LOG_ERR, "ATOMIC_AGGREGATE attribute must not be flagged as \"partial\" (%u)", flag);
+    bgp_attr_flags_diagnose (peer, BGP_ATTR_ATOMIC_AGGREGATE, BGP_ATTR_FLAG_TRANS, flag);
     bgp_notify_send_with_data (peer, BGP_NOTIFY_UPDATE_ERR, BGP_NOTIFY_UPDATE_ATTR_FLAG_ERR, startp, total);
     return -1;
   }
@@ -1325,12 +1345,7 @@
   /* Flag checks. */
   if ((flag & ~BGP_ATTR_FLAG_EXTLEN) != BGP_ATTR_FLAG_OPTIONAL)
   {
-    if (! CHECK_FLAG (flag, BGP_ATTR_FLAG_OPTIONAL))
-      zlog (peer->log, LOG_ERR, "ORIGINATOR_ID attribute must be flagged as \"optional\" (%u)", flag);
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_TRANS))
-      zlog (peer->log, LOG_ERR, "ORIGINATOR_ID attribute must not be flagged as \"transitive\" (%u)", flag);
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_PARTIAL))
-      zlog (peer->log, LOG_ERR, "ORIGINATOR_ID attribute must not be flagged as \"partial\" (%u)", flag);
+    bgp_attr_flags_diagnose (peer, BGP_ATTR_ORIGINATOR_ID, BGP_ATTR_FLAG_OPTIONAL, flag);
     bgp_notify_send_with_data (peer, BGP_NOTIFY_UPDATE_ERR, BGP_NOTIFY_UPDATE_ATTR_FLAG_ERR, startp, total);
     return -1;
   }
@@ -1364,12 +1379,7 @@
   /* Flag checks. */
   if ((flag & ~BGP_ATTR_FLAG_EXTLEN) != BGP_ATTR_FLAG_OPTIONAL)
   {
-    if (! CHECK_FLAG (flag, BGP_ATTR_FLAG_OPTIONAL))
-      zlog (peer->log, LOG_ERR, "CLUSTER_LIST attribute must be flagged as \"optional\" (%u)", flag);
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_TRANS))
-      zlog (peer->log, LOG_ERR, "CLUSTER_LIST attribute must not be flagged as \"transitive\" (%u)", flag);
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_PARTIAL))
-      zlog (peer->log, LOG_ERR, "CLUSTER_LIST attribute must not be flagged as \"partial\" (%u)", flag);
+    bgp_attr_flags_diagnose (peer, BGP_ATTR_CLUSTER_LIST, BGP_ATTR_FLAG_OPTIONAL, flag);
     bgp_notify_send_with_data (peer, BGP_NOTIFY_UPDATE_ERR, BGP_NOTIFY_UPDATE_ATTR_FLAG_ERR, startp, total);
     return -1;
   }
@@ -1412,12 +1422,7 @@
   /* Flag checks. */
   if ((flag & ~BGP_ATTR_FLAG_EXTLEN) != BGP_ATTR_FLAG_OPTIONAL)
   {
-    if (! CHECK_FLAG (flag, BGP_ATTR_FLAG_OPTIONAL))
-      zlog (peer->log, LOG_ERR, "MP_REACH_NLRI attribute must be flagged as \"optional\" (%u)", flag);
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_TRANS))
-      zlog (peer->log, LOG_ERR, "MP_REACH_NLRI attribute must not be flagged as \"transitive\" (%u)", flag);
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_PARTIAL))
-      zlog (peer->log, LOG_ERR, "MP_REACH_NLRI attribute must not be flagged as \"partial\" (%u)", flag);
+    bgp_attr_flags_diagnose (peer, BGP_ATTR_MP_REACH_NLRI, BGP_ATTR_FLAG_OPTIONAL, flag);
     bgp_notify_send_with_data (peer, BGP_NOTIFY_UPDATE_ERR, BGP_NOTIFY_UPDATE_ATTR_FLAG_ERR, startp, total);
     return -1;
   }
@@ -1559,12 +1564,7 @@
   /* Flag checks. */
   if ((flag & ~BGP_ATTR_FLAG_EXTLEN) != BGP_ATTR_FLAG_OPTIONAL)
   {
-    if (! CHECK_FLAG (flag, BGP_ATTR_FLAG_OPTIONAL))
-      zlog (peer->log, LOG_ERR, "MP_UNREACH_NLRI attribute must be flagged as \"optional\" (%u)", flag);
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_TRANS))
-      zlog (peer->log, LOG_ERR, "MP_UNREACH_NLRI attribute must not be flagged as \"transitive\" (%u)", flag);
-    if (CHECK_FLAG (flag, BGP_ATTR_FLAG_PARTIAL))
-      zlog (peer->log, LOG_ERR, "MP_UNREACH_NLRI attribute must not be flagged as \"partial\" (%u)", flag);
+    bgp_attr_flags_diagnose (peer, BGP_ATTR_MP_UNREACH_NLRI, BGP_ATTR_FLAG_OPTIONAL, flag);
     bgp_notify_send_with_data (peer, BGP_NOTIFY_UPDATE_ERR, BGP_NOTIFY_UPDATE_ATTR_FLAG_ERR, startp, total);
     return -1;
   }