The L2NAT Enigma: Doing NAT with only L2 switches

Hola Amigos! Welcome to the first entry in what I hope will become a series where we dive into network protocols, topologies, and specialized solutions tailored for industrial networks. Today, we're looking at a Cisco technology that initially threw me for a loop: Layer 2 Network Address Translation, or L2NAT.

When I first stumbled upon L2NAT, I must admit, I was baffled. My initial guess was that it involved MAC address translation, something along the lines of:

interface Ethernet0
 mac address translation enable
 mac address translation rule mac-translation-table 0011.2233.4455 0055.6677.8899
!
interface Ethernet1
 mac address translation enable
 mac address translation rule mac-translation-table 0055.6677.8899 0011.2233.4455        

However, it turns out that L2NAT is not about translating MAC addresses at all. In fact, it's much closer to the type of NAT you might find at Layer 3—yes, the regular old NAT/PAT we're all more familiar with. I still have beef with the name, but here we are.

Understanding L2NAT

Before diving into the specifics of Layer 2 Network Address Translation, or L2NAT, let's first grasp the basics of traditional Network Address Translation or NAT. Traditionally considered a Layer 3 process, NAT functions by translating one (or many) IP address into another. This process is crucial today, enabling private IP networks (RFC1918) to connect to public networks using a single public IP address. However, Cisco's L2NAT deviates from this standard by operating within the Layer 2 domain—essentially at the network architecture data link layer. In a nutshell, L2NAT facilitates one-to-one IP address translation without needing a traditional Layer 3 device such as a router or firewall.

Real-World Applications of L2NAT

Industrial Networking Challenges: Consider a manufacturing plant where each assembly line is preconfigured with a specific set of IP addresses and operates on internal networks. Often, these configurations are proprietary, making it difficult or impossible for asset owners to reconfigure them. L2NAT allows these assets to be "exposed" to the external network as needed. Interestingly, each assembly line could operate with identical IP configurations without causing conflicts, thanks to L2NAT's one-to-one IP translation capability.

Security Through Segmentation: One of the significant advantages of L2NAT is the ability to improve security without traditional firewalls. Since many industrial systems operate independently, L2NAT can be used to segment networks effectively. By only allowing specific assets to communicate outside their local network—as part of the NAT configuration—it's possible to maintain stringent control over network traffic, reducing the risk of external threats.

Legacy System Integration: Another practical application of L2NAT emerges in dealing with legacy systems. For instance, you might encounter a scenario where you need to connect a device from an outdated system no longer supported by its vendor. The original implementer may have retired years ago, leaving little to no documentation on integrating this system with newer technologies. With L2NAT, you can simply place a Cisco IE switch, assign an appropriate IP to the legacy system, and enable communication without modifying the existing, delicate configuration of the legacy system.

Configuring L2NAT

In this section, we will explore how to configure L2NAT using a Cisco Industrial Ethernet switch. The goal is to enable seamless communication between devices with identical IP addresses across different production lines, all within the same Layer 2 domain. This configuration ensures that specific devices can communicate with a centralized management area without the need for complex routing.

Lab Setup Overview

The lab setup involves two production lines, each with devices assigned the IP address 192.168.1.20. We'll configure L2NAT to translate these identical IPs so that they can communicate externally without IP conflicts. Additionally, one device will be configured to communicate with a management station IP 172.16.1.69, showcasing L2NAT's capability to handle traffic across the network.

This diagram illustrates the setup:

• Line 1: Translates IP 192.168.1.20 internally to 172.16.1.21 externally.
• Line 2: Uses the same internal IP (192.168.1.20), translated to 172.16.1.22 externally.
• Management: Communicates with 172.16.1.69.

Configuring L2NAT: A Step-by-Step Guide

1. Enter Global Configuration Mode:

Switch_Line01#configure terminal        

2. Create and Configure an L2NAT Instance:

Specify the internal and external IP translations for each production line. This configuration allows each line to use the same internal IP address by mapping it to a unique external IP address.

Switch_Line01(config)#l2nat instance L2NAT-LAB
Switch_Line01(config-l2nat)#inside from host 192.168.1.20 to 172.16.1.21
Switch_Line01(config-l2nat)#outside from host 172.16.1.69 to 192.168.1.69        

Switch_Line02(config)#l2nat instance L2NAT-LAB
Switch_Line02(config-l2nat)#inside from host 192.168.1.20 to 172.16.1.22
Switch_Line02(config-l2nat)#outside from host 172.16.1.69 to 192.168.1.69        

3. Apply L2NAT to VLANs:

Apply the L2NAT configuration to the relevant interfaces or VLANs. In this scenario, ensure all configurations are applied to the interfaces connecting to the production lines and management stations.

Switch_Line01(config)#interface GigabitEthernet1/1
Switch_Line01(config-if)#l2nat instance L2NAT-LAB vlan 10        

4. Verify the Configuration:

After applying the configurations, verify that the L2NAT settings are correctly implemented and functioning as expected.

Switch_Line01#show l2nat instance L2NAT-LAB        

Switch_Line01#show l2nat statistics        

5. Important Considerations:

This feature is specific to Cisco Industrial Ethernet Switches and Rockwell Stratix switches running a minimum Cisco IOS version 15.1.

Ensure devices within the L2NAT configuration are on the same VLAN, as routing is not enabled, and the configuration remains purely at Layer 2.

Conclusion

We've explored the capabilities of Layer 2 Network Address Translation and its impact on industrial networks. By enabling devices with identical IP addresses to communicate seamlessly across different segments, L2NAT simplifies network architecture while enhancing security and connectivity.

I'm curious to hear your thoughts on this topic. Should we explore what happens when L2NAT is implemented in a ring topology? Or would you be interested in other industrial network protocols like PDH, SDH, or OPGW? Your feedback will help guide our next steps in this series.

Muchas gracias for joining me in this discussion. Let me know in the comments what you'd like to explore next. Hasta la próxima, amigos!