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Dynamic switch port

As explained in Acing the CCNA Exam: Volume 2 Advanced Networking and Security

A dynamic switch port is a port whose access or trunk status is negotiated by DTP rather than explicitly configured.

A dynamic switch port uses DTP to negotiate whether it operates as an access or trunk port. Because its mode is not explicitly configured, Cisco IOS rejects Port Security on the interface. section 12.1.2 section 12.3

Before enabling Port Security:

  • Configure the interface explicitly with switchport mode access or switchport mode trunk. section 12.1.2
  • Enable Port Security afterward with switchport port-security. section 12.1.2
  • Verify the resulting interface settings with show port-security interface interface. section 12.3
SW1(config)# interface range f0/1-2
SW1(config-if-range)# switchport port-security       #1
Command rejected: FastEthernet0/1 is a dynamic port. #2
#1 Enables Port Security
#2 The command is rejected.

The CLI transcript shows IOS rejecting Port Security on a dynamic port, then accepting it after access mode is configured. section 12.1.2

How Acing the CCNA Exam: Volume 2 Advanced Networking and Security covers it

How deep it goes
Defines the configuration state and its Port Security restriction, without detailing DTP negotiation states.
Examples
Demonstrates switchport port-security failing until switchport mode access explicitly sets the interface mode.
What it assumes
Assumes interface configuration knowledge and distinguishes access mode from trunk mode.
What you can do after
Configure an explicit access or trunk mode before applying Port Security.
How it connects
Connects DTP with access ports, trunk ports, and Port Security.
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Related concepts

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