diff --git a/isisd/iso_checksum.c b/isisd/iso_checksum.c
index e65f6ef..eabe281 100644
--- a/isisd/iso_checksum.c
+++ b/isisd/iso_checksum.c
@@ -42,7 +42,6 @@
  * Verifies that the checksum is correct.
  * Return 0 on correct and 1 on invalid checksum.
  * Based on Annex C.4 of ISO/IEC 8473
- * FIXME: Check for overflow 
  */
 
 int
@@ -52,7 +51,7 @@
   u_int32_t c0;
   u_int32_t c1;
   u_int16_t checksum;
-  int i;
+  int i, partial_len;
 
   p = buffer;
   checksum = 0;
@@ -77,14 +76,21 @@
   c0 = 0;
   c1 = 0;
 
-  for (i = 0; i < len; i++)
+  while (len)
     {
-      c0 = c0 + *(p++);
-      c1 += c0;
-    }
+      partial_len = MIN(len, 5803);
 
-  c0 = c0 % 255;
-  c1 = c1 % 255;
+      for (i = 0; i < partial_len; i++)
+	{
+	  c0 = c0 + *(p++);
+	  c1 += c0;
+	}
+
+      c0 = c0 % 255;
+      c1 = c1 % 255;
+
+      len -= partial_len;
+    }
 
   if (c0 == 0 && c1 == 0)
     return 0;
@@ -96,9 +102,6 @@
  * Creates the checksum. *csum points to the position of the checksum in the 
  * PDU. 
  * Based on Annex C.4 of ISO/IEC 8473
- * we will not overflow until about length of 6000,
- * which is the answer to (255+255n)*n/2 > 2^32
- * so if we have a length of over 5000 we will return zero (for now)
  */
 #define FIXED_CODE
 u_int16_t
@@ -113,7 +116,7 @@
   u_int32_t c1;
   u_int16_t checksum;
   u_int16_t *csum;
-  int i;
+  int i, init_len, partial_len;
 
   checksum = 0;
 
@@ -123,32 +126,34 @@
   csum = (u_int16_t *) (buffer + n);
   *(csum) = checksum;
 
-  /* for the limitation of our implementation */
-  if (len > 5000)
-    {
-      return 0;
-    }
-
   p = buffer;
   c0 = 0;
   c1 = 0;
+  init_len = len;
 
-  for (i = 0; i < len; i++)
+  while (len != 0)
     {
-      c0 = c0 + *(p++);
-      c1 += c0;
+      partial_len = MIN(len, 5803);
+
+      for (i = 0; i < partial_len; i++)
+	{
+	  c0 = c0 + *(p++);
+	  c1 += c0;
+	}
+
+      c0 = c0 % 255;
+      c1 = c1 % 255;
+
+      len -= partial_len;
     }
 
-  c0 = c0 % 255;
-  c1 = c1 % 255;
-
-  mul = (len - n) * (c0);
+  mul = (init_len - n)*(c0);
 
 #ifdef FIXED_CODE
   x = mul - c0 - c1;
   y = c1 - mul - 1;
 
-  if (y >= 0)
+  if (y > 0)
     y++;
   if (x < 0)
     x--;
@@ -159,11 +164,9 @@
   if (x == 0)
     x = 255;
   if (y == 0)
-    y = 255;
+    y = 1;
 
-  x &= 0x00FF;
-
-  checksum = ((y << 8) | x);
+  checksum = (y << 8) | (x & 0xFF);
 
 #else
   x = mul - c0 - c1;