USG FLEX H Series - Session Control

Zyxel_Richard Posts: 249  Zyxel Employee
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edited May 17 in Networking

USG FLEX H Series - Session Control


Session control is a feature designed to manage the number of concurrent sessions a client can initiate through the firewall. This is particularly useful for preventing clients from consuming excessive session resources, which can impact the performance and availability of network services for other users.

Importance of Session Control

Firewalls have a finite number of sessions they can handle at any given time. When a single client or a few clients initiate too many sessions, it can lead to session exhaustion, affecting the ability of other clients to access network resources. Session control helps mitigate this by limiting the number of concurrent sessions each client can create.

Default Settings and Configuration

Default Session Limit

  • The default value for session control is 2000 sessions per client.
  • This limit includes both TCP and UDP sessions.

Configuring Session Control

  • Navigate to Security Policy:
  • Go to Security Policy > Session Control.
  • Set Session Limit:
    • The default limit is 2000 sessions. Adjust this value as needed based on your network requirements.
  • Apply and Save:
    • After setting the desired session limit, save the configuration.

Example Use Case

  • High Session Usage: Applications like torrent clients or peer-to-peer (P2P) software often create a high number of sessions. Limiting the sessions for such applications can prevent a single client from overwhelming the firewall's session capacity.

Monitoring Session Limits

Event Logs:

  • Go to Logs > Event Logs.
  • Filter by the category Session Control.
  • Check for Exceeded Limits:
  • Look for entries indicating that the maximum session limit has been exceeded.
    • Example log entry: Maximum sessions per host was exceeded.

User Impact

  • Symptoms: If a client exceeds the session limit, they will be unable to initiate new sessions.
  • Troubleshooting: If users report issues accessing websites or services, check the session control logs to determine if they have hit the session limit.

Practical Tips

  • Monitoring and Adjustment: Regularly monitor session usage and adjust limits as needed. Some clients may require higher limits due to legitimate needs.
  • Balance: Find a balance between limiting sessions to prevent abuse and ensuring legitimate applications can function correctly.


Session control is a crucial feature for managing the number of concurrent sessions a client can create on the USG FLEX H Series firewall. By setting appropriate session limits, you can ensure fair resource allocation and prevent individual clients from overwhelming the firewall's session capacity. Monitoring and adjusting these limits based on usage patterns will help maintain optimal network performance.