Please share more info about your setup.
Have you configured NDMP devices and STU as per the NDMP Admin Guide  and NAS Appliance Info doc?

yes i have successfully taken the backup before

but the actual error is client connection refused.

i have checked the  ./set_ndmp_attr -verify iacfiler

Connecting to host "iacfiler" as user "root"...
Waiting for connect notification message...
Opening session--attempting with NDMP protocol version 4...
Opening session--successful with NDMP protocol version 4
  host supports TEXT authentication
  host supports MD5 authentication
Getting MD5 challenge from host...
Logging in using MD5 method...
ndmp_connect_client_auth failed
set_ndmp_attr: host "iacfiler" failed
NDMP failed to verify host

regards
hamza

May be worth "resetting" the authentication using set_ndmp_attr  -auth (may have to -delete & -insert?)

The password of filer was changed thats why it was not authenticating but

root@hrsrv01:/usr/openv/volmgr/bin # ./set_ndmp_attr -verify iacfiler

Connecting to host "iacfiler" as user "root"...
Waiting for connect notification message...
Opening session--attempting with NDMP protocol version 4...
Opening session--successful with NDMP protocol version 4
  host supports TEXT authentication
  host supports MD5 authentication
Getting MD5 challenge from host...
Logging in using MD5 method...
Host info is:
  host name "iacfiler"
  os type "NetApp"
  os version "NetApp Release 7.0.7"
  host id "0101187939"
Login was successful
Host supports LOCAL backup/restore
Host supports 3-way backup/restore
Host has SnapVault Secondary license installed
Opening SCSI device "mc0"...

set_ndmp_attr: host "iacfiler" robot "mc0" failed
set_ndmp_attr: one or more robot inquiries failed

NDMP failed to verify host

Regards,
Hamza

Or maybe even trash & recreate robot. :S

Can you run the following on your filer? storage show mc  (this should show any medium changers on your filer, good for reference if nothing else)

((As an aside, there's also 'storage show tape' which will show similar info for any drive set up/visible on the filer))

Not sure if the following will produce output, but you could give it a try: set_ndmp_attr -probe iacfiler (if it works this will show any devices set up - robots & tapes - on your filer, including serial numbers & aliases (eg your robot mc0) )

This won't solve the issue but may give more info to work with (& give us some time to think of something else to try!)

The filer is showing information about the robot and the tapes

iacfiler> storage show mc
storage show mc

Medium Changer:     0e.4L1
Description:        IBM      3573-TL
Serial Number:      00L2U78D8524_LL0
World Wide Name:    L1
Alias Name(s):      mc0
Device State:       available

root@hrsrv01:/usr/openv/volmgr/bin #  ./set_ndmp_attr -probe

Probe Host name: iacfiler
Host "iacfiler" SCSI device model "IBM     3573-TL         ":
  Device "mc0" attributes=0x0
    ELECTRICAL_NAME=0e.4L1
    SERIAL_NUMBER=00L2U78D8524_LL0
    WORLD_WIDE_NAME=L1
    ALIAS 0=mc0
Host "iacfiler" tape device model "IBM     ULT3580-TD3     ":
  Device "rst1l" attributes=(0x5) REWIND RAW
    DENSITY=LTO rd only 200GB cmp  
    ELECTRICAL_NAME=0e.4
    SERIAL_NUMBER=1210337337
    WORLD_WIDE_NAME=
    ALIAS 0=st1
  Device "nrst1l" attributes=(0x4) RAW
    DENSITY=LTO rd only 200GB cmp  
    ELECTRICAL_NAME=0e.4
    SERIAL_NUMBER=1210337337
    WORLD_WIDE_NAME=
    ALIAS 0=st1
  Device "urst1l" attributes=(0x6) UNLOAD RAW
    DENSITY=LTO rd only 200GB cmp  
    ELECTRICAL_NAME=0e.4
    SERIAL_NUMBER=1210337337
    WORLD_WIDE_NAME=
    ALIAS 0=st1
  Device "rst1m" attributes=(0x5) REWIND RAW
    DENSITY=LTO 2 400GB cmp        
    ELECTRICAL_NAME=0e.4
    SERIAL_NUMBER=1210337337
    WORLD_WIDE_NAME=
    ALIAS 0=st1
  Device "nrst1m" attributes=(0x4) RAW
    DENSITY=LTO 2 400GB cmp        
    ELECTRICAL_NAME=0e.4
    SERIAL_NUMBER=1210337337
    WORLD_WIDE_NAME=
    ALIAS 0=st1
  Device "urst1m" attributes=(0x6) UNLOAD RAW
    DENSITY=LTO 2 400GB cmp        
    ELECTRICAL_NAME=0e.4
    SERIAL_NUMBER=1210337337
    WORLD_WIDE_NAME=
    ALIAS 0=st1
  Device "rst1h" attributes=(0x5) REWIND RAW
    DENSITY=LTO 3 400GB            
    ELECTRICAL_NAME=0e.4
    SERIAL_NUMBER=1210337337
    WORLD_WIDE_NAME=
    ALIAS 0=st1
  Device "nrst1h" attributes=(0x4) RAW
    DENSITY=LTO 3 400GB            
    ELECTRICAL_NAME=0e.4
    SERIAL_NUMBER=1210337337
    WORLD_WIDE_NAME=
    ALIAS 0=st1