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Limiting 802.1X Authentication Sessions

Understand how 802.1X authentication session limits cap the number of concurrently active authenticated sessions, using global and per-interface maximums to protect system resources, with logging that identifies which limit triggered a rejection.

802.1X authentication session limits let you cap the number of concurrently active authenticated sessions that the 802.1X authenticator maintains. Limiting sessions helps protect authentication processing and system resources during spikes in authentication activity. You can enforce a global maximum across all interfaces and a per-interface maximum. When the device reaches a configured limit, it rejects additional authentication attempts and writes a system log message that indicates whether the global or per-interface limit caused the rejection.

The effective per-interface limit follows this precedence: an interface-specific override, then the system per-interface default, then an effective default of 100 sessions per interface if neither value is configured. Operational commands such as show dot1x interface detail display the effective limit per interface for verification and troubleshooting.

Benefits of 802.1X authentication session limits

  • Helps prevent resource exhaustion by limiting growth in concurrent authenticated sessions during authentication spikes.

  • Reduces the impact of a single interface with high endpoint density or unexpected behavior by capping sessions per interface.

  • Improves capacity planning by enforcing explicit ceilings for authenticated endpoints per interface and across the device.

  • Speeds fault isolation by indicating in system logs whether the global or per-interface limit caused an authentication denial.

  • Supports verification and auditing by showing effective per-interface limits in operational output.

Overview

When you enable 802.1X authentication session limits, the authenticator enforces hard caps on concurrently active authenticated sessions per interface and, optionally, across the device. As endpoints attempt to authenticate using 802.1X and other authentication methods handled by the authenticator, the device increments the active-session count. When a limit is reached, the device rejects subsequent authentication attempts instead of allocating additional session state.

You control session limiting with settings under protocols dot1x authenticator:

  • global-auth-session-limit enforces a device-wide ceiling across all interfaces. When the total number of active authenticated sessions reaches the configured value, the device denies additional authentication attempts regardless of ingress interface.

  • The per-interface limit is derived using this precedence:

    1. interface <interface-name> auth-session-limit (interface override)

    2. per-interface-session-limit (system default per-interface limit)

    3. Effective default of 100 sessions per interface (when neither value is configured)

Changing limits can affect service. If you reduce an interface auth-session-limit below the current number of active sessions on that interface, the device clears sessions on that interface and then allows reauthentication only up to the new limit. If you reduce global-auth-session-limit below the current device-wide active session count, the device clears authenticated sessions across the device and then enforces the new global maximum. Use show dot1x interface detail to confirm the effective per-interface limit.

Configuration and operational signals

Configure session limits under protocols dot1x authenticator:

  • Set the global limit: set protocols dot1x authenticator global-auth-session-limit <limit>

  • Set the system default per-interface limit: set protocols dot1x authenticator per-interface-session-limit <limit>

  • Override the limit for a specific interface: set protocols dot1x authenticator interface <interface-name> auth-session-limit <limit>

When the device denies an authentication attempt because a limit is reached, system log messages indicate which limit was exceeded:

  • DOT1XD_GLOBAL_SESSION_LIMIT_EXCEEDED indicates that the global session limit blocked the attempt.

  • DOT1XD_INTF_SESSION_LIMIT_EXCEEDED indicates that the per-interface session limit blocked the attempt on a specific interface.

Use these system log messages with show dot1x interface detail to determine whether a single interface is at capacity or whether the device-wide limit is being reached.