Have you recently upgraded your Cisco Catalyst 9300 or 9400 switch to IOS XE 17.9.6 and found that your IoT devices suddenly lost connectivity or failed to obtain IP addresses? You are not alone—this issue is affecting enterprises worldwide, and it’s not your configuration. This is a known software bug in IOS XE 17.9.6, now officially withdrawn by Cisco. 3-Step Fix


This guide will help you diagnose the problem, understand the root cause, and walk you through proven solutions, with extra insights, troubleshooting scripts, and best practices.

cisco iot vlan stopped working after upgrade to 17.09.06 CSCwm57734

1. Common Symptoms

Typical Failure Scenarios

  • IoT devices cannot get IP addresses via DHCP, even though 802.1X authentication seems successful.
  • No network traffic passes through affected ports, even with static IP configuration.
  • show authentication sessions shows “Authorized” and correct VLAN, but devices remain offline.
  • show mac address-table lists client MAC addresses as STATIC, even for dynamically assigned VLANs.
  • Switch logs show DHCP request/offer packets being dropped.
  • Dot1x auth fail vlan can’t assign IP with dhcp

2. What’s Causing the Problem? — Cisco Bug CSCwm57734 Explained

Cisco IOS XE 17.9.6 (Cupertino-17.9.6) for Catalyst 9000 switches, including 9300 and 9400, contains a critical bug identified as CSCwm57734.

Key points(NAC/Dot1x):

  • Affects 802.1X (dot1x) authentication with dynamic VLAN assignment.
  • After upgrade, the switch discards DHCP packets for dot1x-authenticated clients.
  • Even if authentication is successful, network traffic is silently dropped.
  • MAC authentication (MAB) devices may work fine, making diagnosis tricky.

3. How to Fix? [Upgrade to a Fixed Version: 17.9.6a or Later]

Step 1: Prepare for the Upgrade

Ensure Console Access:
Connect to the switch using a console cable (RJ-45 or USB) and a terminal program (such as PuTTY or Tera Term) to prevent lockout if problems occur during the upgrade.

Backup Configuration:
Save your running configuration.

# Save your running configuration.

copy running-config startup-config

# Optionally, also backup to an external USB drive or TFTP server:

copy running-config usbflash0:/your_config_backup.cfg

Download the Correct Image:
Go to Cisco Software Download Center and download the IOS XE 17.9.6a or a later stable version for your switch model (e.g., cat9k_iosxe.17.09.06a.SPA.bin).
Place the .bin file on a reachable TFTP, FTP, SCP server, or USB drive.

Step 2: Upgrade the Switch

Clean Up Inactive Files:

# Free up space by removing old installation files:

install remove inactive

# Confirm with y when prompted.

Copy the New Image to Flash:

# From TFTP:

copy tftp://<server-ip>/cat9k_iosxe.17.09.06a.SPA.bin flash:

# Or from USB:

copy usbflash0:/cat9k_iosxe.17.09.06a.SPA.bin flash:

Verify Image Integrity:
(Highly recommended) Check the MD5 hash to confirm the image is not corrupted:

verify /md5 flash:cat9k_iosxe.17.09.06a.SPA.bin

Set Boot Variable and Save:

conf t
boot system flash:packages.conf
no boot manual
end
write memory
show boot

Install and Activate the New Image:

# This process will reload the switch automatically.

install add file flash:cat9k_iosxe.17.09.06a.SPA.bin activate commit

# Confirm any prompts with y.
# The switch will reboot (expect 15–25 minutes for the process).

Step 3: Verify the Upgrade and Restore Services

Check Software Version:

# After the switch boots, confirm the upgrade succeeded:

show version

Make sure the displayed IOS XE version is 17.9.6a or later.

Test VLAN and DHCP Functionality:

# Verify that IoT devices on the affected VLANs now receive DHCP addresses.

Use show authentication sessions and show mac address-table to confirm correct port authorization and traffic flow.

Monitor the Network:

4. Temporary Workarounds

Only use as a last resort; revert once a fixed image is available.

  • Remove 802.1X authentication from affected ports (security trade-off!).
  • Switch to Open or Low Impact NAC modes for critical devices.
  • Warning: These workarounds can reduce network security—only use in isolated or non-critical environments.

5. How to Identify If You’re Affected by the Cisco IOS XE 17.9.6 VLAN Bug

Key Troubleshooting Commands:

# Check 802.1X Authentication Sessions:
show authentication sessions interface <interface-id> details

# Display DHCP-Related Logs
show logging | include DHCP

# Review MAC Address Table
show mac address-table interface <interface-id>

# Interface Traffic Stats
show interface <interface-id>

# VLAN Assignment Verification
show vlan brief

# Advanced Debug for 802.1X
debug dot1x all
Symptom Comparison Table

Symptom

Normal State

IOS XE 17.9.6 Bug State

802.1X Session Status

Authorized

Authorized

DHCP IP Assignment

Success

Fails

Static IP Traffic

Success

Fails

MAC Table Entry

Dynamic

Static (incorrect)

show interface Traffic Counters

Non-zero, increasing

Zero

FAQ

This is a known software bug (CSCwm57734) in IOS XE 17.9.6 that disrupts dot1x VLAN assignment and DHCP, mainly on Cisco Catalyst 9300 and 9400 switches. Upgrading to IOS XE 17.9.6a or later is recommended.

Common signs include IoT devices failing to get IP addresses, zero network traffic on authenticated ports, and static MAC entries, even when authentication appears successful.

The permanent fix is to upgrade your switch firmware to IOS XE 17.9.6a or any newer stable version, where the bug is resolved.

You can disable dot1x authentication or use “Open” mode on affected ports to temporarily restore connectivity, but this reduces network security and should be used cautiously.

All dot1x-authenticated clients using dynamic VLAN assignment can be affected, not just IoT devices. Devices using MAC authentication bypass may remain operational.

Refer to Cisco’s official bug tracker (CSCwm57734), release notes, and support portal for the latest information and recommended actions.

For robust network solutions and comprehensive technical support, Layer23-Switch.com is your trusted ICT partner.

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