NetBackup port between master for AIR

I agree - MSDP ports are not needed for DataDomain.

DD port requirements are covered under a separate topic:

DataDomain OpenStorage ports
The following ports must be open to use a DataDomain OST storage server.
■ The TCP ports for 2049 (nfs), 111 (portmapper), and 2052 (mountd) must be open from the media server to the target storage server.
■ The UDP port 111 (portmapper) must be open from the media server to the target storage server.
■ The TCP port 2051 (replication) must also be open from the media server to the storage server for optimized duplication.

Port 1556 between masters is a must.

Port 2051 is required between the two data domains for "Managed File Replication" (ost controlled replication).

All Data Domain ports is listed here:

https://community.emc.com/docs/DOC-33258

MFR replication background here:

https://www.emc.com/collateral/software/white-papers/h7082-data-domain-replicator-wp.pdf

As Marianne pointed out 1556 is mandatory between two Netbackupdomains doing AIR via Data Domain. Besides that no other are required.

I agree, but if pbx is mandatory, do you know which way the connections are open between the two M/aster and the two Data Domain.

...because if I replicate from a sensitive site to a less sensitive site, I only allowsto open IP connections from the sensitive site to a less sensitive site.

I do not found this information in the NetBackup documentation.

Even thou you could get away with opening in one direction, you would need the option to do "reverse air" requiring 1556 opened bi-directional.