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Question

Difficulty: MediumLayer 2 Security Features (Port Security, DHCP Snooping, DAI)

A network administrator configures an access port on a Cisco Catalyst switch using the commands `switchport port-security` and `switchport port-security mac-address sticky`. A host connects to the port, and its MAC address is dynamically learned and written to the running configuration. Shortly after, the switch experiences an unexpected reboot due to a brief power failure, before `copy running-config startup-config` or `write memory` was executed. What happens to the learned sticky MAC address on this interface after the switch completes its boot sequence?

  1. The learned sticky MAC address is lost from memory, and the switch port dynamically learns the MAC address of the first frame received post-reboot.Answer
  2. B
    The sticky MAC address automatically persists in NVRAM across reboots without needing a manual configuration save command.
  3. C
    The interface enters an err-disabled state immediately upon booting because the unmapped MAC address conflicts with native VLAN tagging.
  4. D
    The switch sends a TACACS+/RADIUS request to restore the pre-reboot sticky MAC table from the centralized authentication server.

Answer

The learned sticky MAC address is lost from RAM, causing the port to dynamically re-learn the MAC address of the next device that transmits frames on the link.
Enabling sticky MAC learning (`switchport port-security mac-address sticky`) automatically adds dynamically learned MAC addresses into the volatile `running-config` stored in RAM. If the switch reboots before `copy running-config startup-config` is executed, the learned MAC entries are lost, and the port will learn the MAC address of the next frame transmitted post-reboot.

Step-by-Step Solution

1
Analyze how sticky MAC address learning operates in Cisco IOS.
When `switchport port-security mac-address sticky` is enabled, dynamically learned MAC addresses are converted into static-like entries added directly into the switch's `running-config` in RAM.
This avoids manual MAC address entry while retaining MAC restrictions during current uptime.
2
Evaluate the effect of a switch reboot without saving the running configuration.
Because RAM is volatile memory, any modifications to `running-config` that were not saved to `startup-config` in NVRAM (via `copy running-config startup-config`) are cleared upon reboot.
The startup configuration restored at boot lacks the dynamically added sticky MAC statement.
3
Determine the state of the interface after rebooting.
The port remains operational with sticky port security enabled, but its learned sticky MAC list is empty, allowing it to learn a new sticky MAC address from incoming frames.
The underlying port security feature configuration remains in startup-config, but the learned operational data was lost.

Key Concept

Port Security Sticky MAC Address Persistence
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Topics

Layer 2 Security Features (Port Security, DHCP Snooping, DAI)

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