Differential backups via NDMP are always fixed by Level 1.

Problem

Explanation:
In case of full backups via NDMP, the backup method is NOT selectable, and is always fixed by Level 0.  (Figure 1)

In case of differential backups via NDMP, the backup method is NOT selectable, and is always fixed by Level 1.  (Figure 1)

In case of incremental backups via NDMP, the backup method is selectable from Level 1 to Level 9. (Figure 2)

Figure 1 - Full and Differential


Figure 2 - Incremental



How to view each Level:

In case of full backups, level 0 and the start time are found in job log.

NDMP Log Message: DUMP: Date of this level 0 dump: Tue May 7 02:35:51 2013.
NDMP Log Message: DUMP: Date of last level 0 dump: the epoch.

In case of differential backups, level 0/1 and the start time are found in job log.

<1st differential backup>
NDMP Log Message: DUMP: Date of this level 1 dump: Tue May 7 02:36:49 2013.
NDMP Log Message: DUMP: Date of last level 0 dump: Tue May 7 02:35:51 2013.

<2nd differential backup>
NDMP Log Message: DUMP: Date of this level 1 dump: Tue May 7 02:37:48 2013.
NDMP Log Message: DUMP: Date of last level 0 dump: Tue May 7 02:35:51 2013.

In case of incremental backups, level 0/1/2/... and the start time are found in job log.

<Level 1 - Incremental>
NDMP Log Message: DUMP: Date of this level 1 dump: Tue May 7 02:50:05 2013.
NDMP Log Message: DUMP: Date of last level 0 dump: Tue May 7 02:49:05 2013.

<Level 2 - Incremental>
NDMP Log Message: DUMP: Date of this level 2 dump: Tue May 7 02:50:35 2013.
NDMP Log Message: DUMP: Date of last level 1 dump: Tue May 7 02:50:05 2013.

 

Solution

This is by design. Differential backups are fixed by Level 1 as Full backups are fixed by Level 0.
Level 2 to 9 is available for Incremental backups.