Connecting VIRL to the Outside World

I’ve been leveraging VIRL for some time to build and test self-contained labs. I’ve always known that there was some ability to connect to the world outside of this environment. Recently, I decided to configure this functionality and I wanted to take just a moment to share what I found.

First and foremost, this isn’t anything difficult or time consuming. So if you have a need to leverage physical devices with your VIRL deployment, don’t hesitate before building it out.

There are two mechanisms for outside connectivity. The first mechanism is called SNAT. This method basically builds static NAT in and out of the environment. I get how this could be beneficial, but I would typically prefer to keep any NAT configuration contained to an environment that I am very familiar with (possibly an ASA or IOS instance outside the lab when an additional NAT layer is required).

The second method, and configuration we will be testing is called FLAT. In this configuration, VIRL connects a L2 broadcast domain between a lab device and an Ethernet interface. In my example I am running the VIRL components in a VM environment on ESXi. So this is a virtual interface that needs to be mapped through the VMWare vSwitch.

To test this functionality, I created the following topology.


From a VIRL perspective, there are a couple of things to be aware of. The default configuration would have VIRL owning the subnet and the external default GW existing at

This configuration can be found in User Workspace Management in the VIRL Server -> System Configuration section (in the Networks tab).


Using the above defaults, the only thing that is necessary on the physical (external network devices) is to build out a VLAN that has a default gateway that matches the indicated Gateway IP address. The VM environment needs to be mapped through to the physical network. In my case, I built a VLAN 10 and assigned on a Meraki Firewall. From a VMWare perspective the ‘eth1’ seemed to be presented as ‘Network adapter 2’.

vSwitch Configuration


It is also worth noting that flat networks seem to require promiscuous mode (on the security tab above).


VM Attachment to ‘flat’


At this point a test environment can be built by building a topology with any device connected to the General type ‘L2 External (FLAT)’. In this example, I attached a CSR1000v to ‘flat-1’.


For verification, a simple test can be performed from the csr1000v.

csr1000v-1(config)#interface GigabitEthernet2
csr1000v-1(config-if)#ip address
csr1000v-1(config)#ip route
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/2 ms
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 40/54/95 ms

From a VIRL monitoring standpoint, there is some useful information that is provided in the “Connectivity” section of User Workspace Manager.


My recommendation for putting this into practice is to build out a dedicated VLAN. I would also allow VIRL to own the IP addressing with the exception of the default gateway. If there is a need for other external IP addresses on the connected subnet, I would start with 2-49 in the last octect. Anything else might require adjustment of the pool in the Flat Network settings.


Configuring external connectivity with VIRL isn’t overly complex. There is just a need to think through what the physical and virtual environments should look like and how they relate. This article should serve as a simple starting point for those who have a need for this type of functionality.

Disclaimer: This article includes the independent thoughts, opinions, commentary or technical detail of Paul Stewart. This may or may does not reflect the position of past, present or future employers.


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About Paul Stewart, CCIE 26009 (Security)

Paul is a Network and Security Engineer, Trainer and Blogger who enjoys understanding how things really work. With over 15 years of experience in the technology industry, Paul has helped many organizations build, maintain and secure their networks and systems.
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5 Responses to Connecting VIRL to the Outside World

  1. Uros says:

    Can you connect device to multiple external interfaces? For example, if I have physical FW, can I create two virtual routers and connect those two routers through physical firewall?

    #VIRL [CSR1]# — FW — #VIRL [CSR2]#

    • I haven’t tried this myself, but I think it should be possible. I think that there are two ‘flat’ interfaces by default. You would specify which one to use when you build the topology. If you need additional ones, they can be created as well. I think this video will help you understand-

    • I did attempt this and had success. In the current version, you can add two ‘flat’ networks. These are actually prebuilt using and When you add the ‘flat’ node, you can select the ‘flat-x’ node in design view and open the flat/snat properties (typically at the bottom of the screen). There is a drop down box that allows you to select “flat” or “flat1”. It is obviously necessary to have a properly configure underlay network (and VMNetwork as applicable).

      • Rhea says:

        Hello Paul,

        I am trying to do a similar thing wherein I want to connect my one physical switch with two virtual switches on VIRL. But as I do that I get the error as :

        Flat node “flat-1″ must be connected to one VM node, not 2 (iosvl2-2, iosvl2-1): problem of VIRL file at line 25.

        I would be really grateful if you could advice on how to resolve the error.

        Thank you for your time. Looking forward to your response.

        Rhea Vijh
        [email protected]

      • I have no idea on that one. Someone over at the community may be able to help with that issue.

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