configure border router with a given external route prefix entry

        Args:
            P_Prefix: IPv6 prefix for the route
            P_Stable: is true if the external route prefix is stable network data
            R_Preference: a two-bit signed integer indicating Router preference
                          1: high
                          0: medium
                         -1: low

        Returns:
            True: successful to configure the border router with a given external route prefix
            False: fail to configure the border router with a given external route prefix

def configExternalRouter(self, P_Prefix, P_stable, R_Preference=0):
        """configure border router with a given external route prefix entry

        Args:
            P_Prefix: IPv6 prefix for the route
            P_Stable: is true if the external route prefix is stable network data
            R_Preference: a two-bit signed integer indicating Router preference
                          1: high
                          0: medium
                         -1: low

        Returns:
            True: successful to configure the border router with a given external route prefix
            False: fail to configure the border router with a given external route prefix
        """
        print '%s call configExternalRouter' % self.port
        print P_Prefix
        stable = ''
        prefix = self.__convertIp6PrefixStringToIp6Address(str(P_Prefix))
        try:
            if R_Preference == 1:
                prf = 'high'
            elif R_Preference == 0:
                prf = 'med'
            elif R_Preference == -1:
                prf = 'low'
            else:
                pass

            if P_stable:
                stable += 's'
                cmd = 'route add %s/64 %s %s' % (prefix, stable, prf)
            else:
                cmd = 'route add %s/64 %s' % (prefix, prf)
            print cmd

            if self.__sendCommand(cmd)[0] == 'Done':
                # send server data ntf to leader
                return self.__sendCommand('netdataregister')[0] == 'Done'
        except Exception, e:
            ModuleHelper.WriteIntoDebugLogger("configExternalRouter() Error: " + str(e))