Temporarily out-commented the problematic code and added a comment that describes the problem

core/net/uip-nd6.c

@@ -382,9 +382,20 @@ uip_nd6_periodic(void)
   for(i = 0; i < UIP_CONF_ND6_MAX_DEFROUTERS; i ++) {
     if(uip_nd6_defrouter_list[i].used == 1) {
       router = &(uip_nd6_defrouter_list[i]);
+      
+      /* XXX when run on a platform with a 16-bit clock_time_t, the
+      timer_expired() below causes the default route to be immediately
+      be removed causing communucation problems. We comment it out as
+      a quick-fix to this problem on the Atmel RAven platform, but
+      since this is needed for IPv6 compliance, we will solve the
+      problem by making clock_time_t 32 bits instead after the uIPv6
+      snapshot release.
+	 
       if(timer_expired(&(router->lifetime))) {
 	uip_nd6_defrouter_rm(router);
       }
+
+      */
     }
   }
   /*PERIODIC PROCESSING FOR NEIGHBOR CACHE*/