This guide will provide information on the difference between what agents and "Connect all" does and why we recommend using agents instead of using "Connect all"
What is "Connect All" (Auto Subnet agent)?
The Auto Subnet Agent is a function that allows the SiteManager to connect to a whole subnet, all agents available on the SiteManager or both at once. For more information about configuring "Connect All" (Auto Subnet Agent) Please see: Link
What is an Agent?
A agent is something that you configure in a SiteManager in order for computers (LinkManager) to communicate with specific end devices on specific ports.
Why use an Agent?
The first one is, of course, security. When using "Connect All", you will have access to the entire IP scope on the device side. Resulting in access to all ports on every IP address. Opening access to 16,7 million possible TCP connections might not be what you want. When using the agents, you gain access to only the ports you need for the specific session. This could be 1 address with eg. 5 ports available. If you need access to more than one device at a time, you can group them together. (Read more about grouping agents together here: Link)
The second one is the one wit the most impact for your remote session. The agents are not just a safety measure. They contain functionality and knowledge about the device that is connected. And they counter the lag and instability associated with the internet. If your latency gets below a specific threshold, the device in the other end will time out, and your upload may fail, or your login may be rejected. Siemens is a good example of how the agent counteracts low latency problems. The Siemens Ethernet agent will send a series of packets immediately after connecting to the panel without waiting for the payload from LM side. This ensures that a login to the Siemens panel is possible at all, it would fail on a "normal" VPN connection or with just "Connect All". Beckhoff is another example, they are using "AMS nat", which is used for communication between the PLC and its gateway (not the SiteManager). If the PLC is connected to a gateway, and is making use of the AMS nat protocol, then the SiteManager has to know about this. The option on the agent enables this possibility, and tells the SiteManager to connect to the PLC via a gateway.
To sum up, you can use the "Connect All" option if you have no other option (or just need a quick RDP session to a PC), but for normal use, you should use the agents.