Isilon OneFS: NDMP failure "Unable to establish data connection"

Environmental

(DMA)Avamar backup of a data path on Isilon fails as the DMA is unable to establish a data connection with Isilon nodes.
DMA logs show:

2020-02-20 20:00:00 avndmp Error <41862>: [snapup-/ifs/data/ndmp/data] Unable to establish data connection to address xx.xx.xx.xx port xxxxx.
2020-02-20 20:00:00 avndmp Error <41862>: [snapup-/ifs/data/ndmp/data] Unable to establish data connection to address yy.yy.yy.yy port yyyyy.

Isilon node logs (messages and trace logs) do not show any activity for the session.

Firewall

The data port on Isilon, for NDMP backup operations, is random.
The port cannot be controlled, nor should custom port ranges be configured for backup purposes.
The following commands show that ports are already open on Isilon:

# from the backup server, telnet <ip> <port number>

Using the Isilon IP address and port number from the failures in the backup logs

# from any isilon node, telnet <ip> <port number>

Using the same ip and port info from the previous command

This confirms a block between DMA and Isilon.
DMA support to be engaged to review Avfirewall settings if applicable.
The topic should then be highlighted to the site networking team for further review if needed.

KB469856