Hunting Down Rogue Media Converters on Your Network: A Practical Guide

So, you've been tasked with a network-wide scavenger hunt: finding and disabling all media converters (RJ45 to fiber optic) lurking within your Cisco-powered LAN and WAN. It's a common challenge, especially when aiming for network standardization, security improvements, or simply cleaning up undocumented corners. Let's break down some strategies for flushing out these devices.

Understanding the Mission: Why Disable Media Converters?

Before we dive into the "how," let's briefly touch upon the "why." Media converters, while useful tools for bridging different media types, can sometimes introduce complexities and potential vulnerabilities:

• Increased Points of Failure: Each converter adds another device that could fail, impacting network uptime.
• Security Concerns: Unmanaged converters can potentially be exploited if not properly secured.
• Performance Bottlenecks: Older or lower-quality converters might introduce latency or limit bandwidth.
• Management Overhead: Tracking and managing a large number of converters can be a burden.

Telltale Signs: Identifying Media Converters in the Wild

Your current lead – the "Duplex: a-half" setting on connected interfaces – is a good starting point. Here's a more comprehensive approach to uncovering those converters:

• Leverage the "Duplex: a-half" Clue:

  • Scripting is Your Friend: Write a script (using Python with libraries like Netmiko or Paramiko) to automatically connect to your Cisco switches (9300 and 9200 series) and check the duplex settings of all interfaces.
  • Focus on Edge Ports: Concentrate your search on interfaces connected to end-user devices or areas where fiber connections are less common.
  • Document Your Findings: Create a spreadsheet or database to track the interfaces configured as "a-half" duplex.

• MAC Address Analysis:

  • Identify Converter OUI Ranges: Media converters often have specific OUI (Organizationally Unique Identifier) ranges in their MAC addresses. Research common OUI prefixes associated with media converter manufacturers.
  • MAC Address Table Crawl: Use your Cisco switches' CLI commands (e.g., show mac address-table) to retrieve MAC address tables.
  • Filter and Compare: Filter the MAC address table output for the identified OUI ranges. This will give you a list of MAC addresses likely belonging to media converters.
  • Trace the MACs: Once you have the MAC addresses, trace them back to the connected switch ports using commands like show mac address-table address <MAC_ADDRESS>.

• Network Discovery Tools:

  • Nmap: Use Nmap with OS detection (-O) to scan your network. Media converters might have identifiable operating systems or services running.
  • Network Inventory Software: Consider using network inventory software (e.g., Lansweeper) that can automatically discover and identify devices on your network. These tools often have built-in device recognition capabilities.

• Physical Inspection:

  • The Old-Fashioned Way: Sometimes, the simplest approach is the most effective. Conduct a physical walk-through of your network closets and server rooms, visually inspecting for media converters. Pay close attention to areas where copper Ethernet cables connect to fiber optic cables.

• CDP/LLDP Information:

  • Cisco Discovery Protocol (CDP) and Link Layer Discovery Protocol (LLDP): These protocols can sometimes provide information about connected devices. Use commands like show cdp neighbors or show lldp neighbors on your Cisco switches to see if any connected devices identify themselves as media converters or have unusual device descriptions.

Disabling Media Converters: Proceed with Caution

Once you've identified the media converters, proceed with caution when disabling them:

• Communication is Key: Inform users who might be affected by the change beforehand.
• Scheduled Maintenance Window: Perform the disabling during a scheduled maintenance window to minimize disruptions.
• Backup Plan: Have a rollback plan in case disabling a converter causes unexpected issues.
• Configuration Changes: Depending on your network design, you might need to reconfigure the affected switch ports (e.g., change the duplex settings).

Alternatives to Disabling: Consider Upgrading

Before completely eliminating media converters, consider whether upgrading network infrastructure might be a more strategic long-term solution. Replacing older switches with models that have built-in fiber optic ports can eliminate the need for converters altogether, simplifying your network and improving performance.

Final Thoughts

Finding and disabling media converters can be a tedious task, but by combining these techniques, you can effectively hunt them down and improve the overall management and security of your network. Remember to document your findings and any changes you make to ensure a smooth and well-maintained network environment. Good luck!