diff --git a/lib/checksum.c b/lib/checksum.c
index 3ddde81..43940b7 100644
--- a/lib/checksum.c
+++ b/lib/checksum.c
@@ -51,6 +51,8 @@
 
 /* To be consistent, offset is 0-based index, rather than the 1-based 
    index required in the specification ISO 8473, Annex C.1 */
+/* calling with offset == FLETCHER_CHECKSUM_VALIDATE will validate the checksum
+   without modifying the buffer; a valid checksum returns 0 */
 u_int16_t
 fletcher_checksum(u_char * buffer, const size_t len, const uint16_t offset)
 {
@@ -62,13 +64,14 @@
   
   checksum = 0;
 
-  assert (offset < len);
 
-  /*
-   * Zero the csum in the packet.
-   */
-  csum = (u_int16_t *) (buffer + offset);
-  *(csum) = 0;
+  if (offset != FLETCHER_CHECKSUM_VALIDATE)
+    /* Zero the csum in the packet. */
+    {
+      assert (offset < (len - 1)); /* account for two bytes of checksum */
+      csum = (u_int16_t *) (buffer + offset);
+      *(csum) = 0;
+    }
 
   p = buffer;
   c0 = 0;
@@ -89,7 +92,7 @@
 
       left -= partial_len;
     }
-  
+
   /* The cast is important, to ensure the mod is taken as a signed value. */
   x = (int)((len - offset - 1) * c0 - c1) % 255;
 
@@ -98,17 +101,24 @@
   y = 510 - c0 - x;
   if (y > 255)  
     y -= 255;
-  
-  /*
-   * Now we write this to the packet.
-   * We could skip this step too, since the checksum returned would
-   * be stored into the checksum field by the caller.
-   */
-  buffer[offset] = x;
-  buffer[offset + 1] = y;
 
-  /* Take care of the endian issue */
-  checksum = htons((x << 8) | (y & 0xFF));
+  if (offset == FLETCHER_CHECKSUM_VALIDATE)
+    {
+      checksum = (c1 << 8) + c0;
+    }
+  else
+    {
+      /*
+       * Now we write this to the packet.
+       * We could skip this step too, since the checksum returned would
+       * be stored into the checksum field by the caller.
+       */
+      buffer[offset] = x;
+      buffer[offset + 1] = y;
+
+      /* Take care of the endian issue */
+      checksum = htons((x << 8) | (y & 0xFF));
+    }
 
   return checksum;
 }
diff --git a/lib/checksum.h b/lib/checksum.h
index da1d3cb..b310f74 100644
--- a/lib/checksum.h
+++ b/lib/checksum.h
@@ -1,2 +1,3 @@
 extern int in_cksum(void *, int);
+#define FLETCHER_CHECKSUM_VALIDATE 0xffff
 extern u_int16_t fletcher_checksum(u_char *, const size_t len, const uint16_t offset);
diff --git a/ospf6d/ospf6_flood.c b/ospf6d/ospf6_flood.c
index 670c5d1..b815972 100644
--- a/ospf6d/ospf6_flood.c
+++ b/ospf6d/ospf6_flood.c
@@ -733,7 +733,6 @@
 {
   struct ospf6_lsa *new = NULL, *old = NULL, *rem = NULL;
   int ismore_recent;
-  unsigned short cksum;
   int is_debug = 0;
 
   ismore_recent = 1;
@@ -751,8 +750,7 @@
     }
 
   /* (1) LSA Checksum */
-  cksum = ntohs (new->header->checksum);
-  if (ntohs (ospf6_lsa_checksum (new->header)) != cksum)
+  if (! ospf6_lsa_checksum_valid (new->header))
     {
       if (is_debug)
         zlog_debug ("Wrong LSA Checksum, discard");
diff --git a/ospf6d/ospf6_lsa.c b/ospf6d/ospf6_lsa.c
index e65752d..ff061df 100644
--- a/ospf6d/ospf6_lsa.c
+++ b/ospf6d/ospf6_lsa.c
@@ -29,6 +29,7 @@
 #include "command.h"
 #include "memory.h"
 #include "thread.h"
+#include "checksum.h"
 
 #include "ospf6_proto.h"
 #include "ospf6_lsa.h"
@@ -672,47 +673,36 @@
 
 
 
-/* enhanced Fletcher checksum algorithm, RFC1008 7.2 */
-#define MODX                4102
-#define LSA_CHECKSUM_OFFSET   15
+/* Fletcher Checksum -- Refer to RFC1008. */
 
+/* All the offsets are zero-based. The offsets in the RFC1008 are
+   one-based. */
 unsigned short
 ospf6_lsa_checksum (struct ospf6_lsa_header *lsa_header)
 {
-  u_char *sp, *ep, *p, *q;
-  int c0 = 0, c1 = 0;
-  int x, y;
-  u_int16_t length;
+  u_char *buffer = (u_char *) &lsa_header->type;
+  int type_offset = buffer - (u_char *) &lsa_header->age; /* should be 2 */
 
-  lsa_header->checksum = 0;
-  length = ntohs (lsa_header->length) - 2;
-  sp = (u_char *) &lsa_header->type;
+  /* Skip the AGE field */
+  u_int16_t len = ntohs(lsa_header->length) - type_offset;
 
-  for (ep = sp + length; sp < ep; sp = q)
-    {
-      q = sp + MODX;
-      if (q > ep)
-        q = ep;
-      for (p = sp; p < q; p++)
-        {
-          c0 += *p;
-          c1 += c0;
-        }
-      c0 %= 255;
-      c1 %= 255;
-    }
+  /* Checksum offset starts from "type" field, not the beginning of the
+     lsa_header struct. The offset is 14, rather than 16. */
+  int checksum_offset = (u_char *) &lsa_header->checksum - buffer;
 
-  /* r = (c1 << 8) + c0; */
-  x = ((length - LSA_CHECKSUM_OFFSET) * c0 - c1) % 255;
-  if (x <= 0)
-    x += 255;
-  y = 510 - c0 - x;
-  if (y > 255)
-    y -= 255;
+  return (unsigned short)fletcher_checksum(buffer, len, checksum_offset);
+}
 
-  lsa_header->checksum = htons ((x << 8) + y);
+int
+ospf6_lsa_checksum_valid (struct ospf6_lsa_header *lsa_header)
+{
+  u_char *buffer = (u_char *) &lsa_header->type;
+  int type_offset = buffer - (u_char *) &lsa_header->age; /* should be 2 */
 
-  return (lsa_header->checksum);
+  /* Skip the AGE field */
+  u_int16_t len = ntohs(lsa_header->length) - type_offset;
+
+  return (fletcher_checksum(buffer, len, FLETCHER_CHECKSUM_VALIDATE) == 0);
 }
 
 void
diff --git a/ospf6d/ospf6_lsa.h b/ospf6d/ospf6_lsa.h
index 7d93f5c..263411f 100644
--- a/ospf6d/ospf6_lsa.h
+++ b/ospf6d/ospf6_lsa.h
@@ -237,6 +237,7 @@
 extern int ospf6_lsa_refresh (struct thread *);
 
 extern unsigned short ospf6_lsa_checksum (struct ospf6_lsa_header *);
+extern int ospf6_lsa_checksum_valid (struct ospf6_lsa_header *);
 extern int ospf6_lsa_prohibited_duration (u_int16_t type, u_int32_t id,
                                           u_int32_t adv_router, void *scope);
 
diff --git a/ospfd/ospf_lsa.c b/ospfd/ospf_lsa.c
index 493209a..5579d8e 100644
--- a/ospfd/ospf_lsa.c
+++ b/ospfd/ospf_lsa.c
@@ -192,6 +192,18 @@
   return fletcher_checksum(buffer, len, checksum_offset);
 }
 
+int
+ospf_lsa_checksum_valid (struct lsa_header *lsa)
+{
+  u_char *buffer = (u_char *) &lsa->options;
+  int options_offset = buffer - (u_char *) &lsa->ls_age; /* should be 2 */
+
+  /* Skip the AGE field */
+  u_int16_t len = ntohs(lsa->length) - options_offset;
+
+  return(fletcher_checksum(buffer, len, FLETCHER_CHECKSUM_VALIDATE) == 0);
+}
+
 
 
 /* Create OSPF LSA. */
diff --git a/ospfd/ospf_packet.c b/ospfd/ospf_packet.c
index 351fb21..ede5908 100644
--- a/ospfd/ospf_packet.c
+++ b/ospfd/ospf_packet.c
@@ -1590,7 +1590,7 @@
 
       /* Validate the LSA's LS checksum. */
       sum = lsah->checksum;
-      if (sum != ospf_lsa_checksum (lsah))
+      if (! ospf_lsa_checksum_valid (lsah))
 	{
 	  /* (bug #685) more details in a one-line message make it possible
 	   * to identify problem source on the one hand and to have a better