diff --git a/nest/config.Y b/nest/config.Y
index 48940ffd..e8437770 100644
--- a/nest/config.Y
+++ b/nest/config.Y
@@ -22,7 +22,13 @@ static list *this_p_list;
 static struct password_item *this_p_item;
 static int password_id;
 
-static list *
+static inline void
+reset_passwords(void)
+{
+ this_p_list = NULL;
+}
+
+static inline list *
 get_passwords(void)
 {
   list *rv = this_p_list;
diff --git a/proto/ospf/config.Y b/proto/ospf/config.Y
index 00b6be7c..bfe2d9c8 100644
--- a/proto/ospf/config.Y
+++ b/proto/ospf/config.Y
@@ -19,6 +19,18 @@ static struct iface_patt *this_ipatt;
 static struct nbma_node *this_nbma;
 static struct area_net_config *this_pref;
 
+static void
+finish_iface_config(struct ospf_iface_patt *ip)
+{
+  ip->passwords = get_passwords();
+
+  if ((ip->autype == OSPF_AUTH_CRYPT) && (ip->helloint < 5))
+    log(L_WARN "Hello or poll interval less that 5 makes cryptographic authenication prone to replay attacks");
+
+  if ((ip->autype == OSPF_AUTH_NONE) && (ip->passwords != NULL))
+    log(L_WARN "Password option without authentication option does not make sense");
+}
+
 CF_DECLS
 
 CF_KEYWORDS(OSPF, AREA, OSPF_METRIC1, OSPF_METRIC2, OSPF_TAG)
@@ -83,7 +95,7 @@ ospf_area_item:
  ;
 
 ospf_vlink:
-   ospf_vlink_start '{' ospf_vlink_opts '}' { OSPF_PATT->passwords = get_passwords(); }
+   ospf_vlink_start '{' ospf_vlink_opts '}' { finish_iface_config(OSPF_PATT); }
  | ospf_vlink_start
  ;
 
@@ -121,6 +133,7 @@ ospf_vlink_start: VIRTUAL LINK idval
   OSPF_PATT->type = OSPF_IT_VLINK;
   init_list(&OSPF_PATT->nbma_list);
   OSPF_PATT->autype = OSPF_AUTH_NONE;
+  reset_passwords();
  }
 ;
 
@@ -223,6 +236,7 @@ ospf_iface_start:
   OSPF_PATT->stub = 0;
   init_list(&OSPF_PATT->nbma_list);
   OSPF_PATT->autype = OSPF_AUTH_NONE;
+  reset_passwords();
  }
 ;
 
@@ -237,7 +251,7 @@ ospf_iface_opt_list:
  ;
 
 ospf_iface:
-  ospf_iface_start iface_patt ospf_iface_opt_list { OSPF_PATT->passwords = get_passwords(); }
+  ospf_iface_start iface_patt ospf_iface_opt_list { finish_iface_config(OSPF_PATT); }
  ;
 
 ospf_iface_list:
diff --git a/proto/ospf/ospf.h b/proto/ospf/ospf.h
index 4e37c184..fb78af4e 100644
--- a/proto/ospf/ospf.h
+++ b/proto/ospf/ospf.h
@@ -161,7 +161,8 @@ struct ospf_iface
   u16 autype;
   u16 helloint;			/* number of seconds between hello sending */
   list *passwords;
-  u32 csn;                      /* Crypt seq num. that will be sent net */
+  u32 csn;                      /* Last used crypt seq number */
+  bird_clock_t csn_use;         /* Last time when packet with that CSN was sent */
   ip_addr drip;			/* Designated router */
   u32 drid;
   ip_addr bdrip;		/* Backup DR */
diff --git a/proto/ospf/packet.c b/proto/ospf/packet.c
index c6b233f8..23785fe8 100644
--- a/proto/ospf/packet.c
+++ b/proto/ospf/packet.c
@@ -75,13 +75,25 @@ ospf_pkt_finalize(struct ospf_iface *ifa, struct ospf_packet *pkt)
 
       pkt->checksum = 0;
 
+      /* Perhaps use random value to prevent replay attacks after
+	 reboot when system does not have independent RTC? */
       if (!ifa->csn)
-        ifa->csn = (u32) time(NULL);
+	{
+	  ifa->csn = (u32) now;
+	  ifa->csn_use = now;
+	}
+
+      /* We must have sufficient delay between sending a packet and increasing 
+	 CSN to prevent reordering of packets (in a network) with different CSNs */
+      if ((now - ifa->csn_use) > 1)
+	ifa->csn++;
+
+      ifa->csn_use = now;
 
       pkt->u.md5.keyid = passwd->id;
       pkt->u.md5.len = OSPF_AUTH_CRYPT_SIZE;
       pkt->u.md5.zero = 0;
-      pkt->u.md5.csn = htonl(ifa->csn++);
+      pkt->u.md5.csn = htonl(ifa->csn);
       tail = ((void *)pkt) + ntohs(pkt->length);
       MD5Init(&ctxt);
       MD5Update(&ctxt, (char *) pkt, ntohs(pkt->length));
@@ -184,12 +196,14 @@ ospf_pkt_checkauth(struct ospf_neighbor *n, struct ospf_iface *ifa, struct ospf_
 
       if (n)
       {
-        if(ntohs(pkt->u.md5.csn) < n->csn)
-        {
-          OSPF_TRACE(D_PACKETS, "OSPF_auth: lower sequence number");
-          return 0;
-        }
-        n->csn = ntohs(pkt->u.md5.csn);
+	u32 rcv_csn = ntohl(pkt->u.md5.csn);
+	if(rcv_csn < n->csn)
+	{
+	  OSPF_TRACE(D_PACKETS, "OSPF_auth: lower sequence number (rcv %d, old %d)", rcv_csn, n->csn);
+	  return 0;
+	}
+
+	n->csn = rcv_csn;
       }
 
       MD5Init(&ctxt);