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netlab: Change Stub Networks into Loopbacks

One of the least-documented limitations of virtual networking labs is the number of network interfaces a virtual machine could have. vSphere supports up to 10 interfaces per VM, the default setting for vagrant-libvirt is eight, and I couldn’t find the exact numbers for KVM. Many vendors claim their KVM limit is around 25; I was able to bring up a Nexus 9300v device with 40 adapters.

Anyway, a dozen interfaces should be good enough if you’re building a proof-of-concept fabric, but it might get a bit tight if you want to emulate plenty of edge subnets.

netlab supported stub links (links with a single device connected to them) from the very beginning, and I consciously chose to implement them as LAN interfaces. Who knows what weird features might be available on a LAN interface that you cannot configure on a loopback interface (across over a dozen platforms)? For example, if you want to build a simple 2-node network with LAN and WAN interface, you’ll get Ethernet interfaces for the stub links:

defaults.device: iosv

module: [ ospf ]

nodes: [ r1, r2 ]
links:
- r1-r2
- r1:
- r2:

That works very well until you hit the limit mentioned above, at which point it might be a good idea to turn some of those LAN interfaces into loopbacks.

netlab release 1.5.0 introduced loopback link type. If a link has a single node attached to it and a link type set to loopbacknetlab uses a loopback interface to implement it. For example, the following topology has a P2P link between two routers and an additional loopback interface per router to emulate a LAN network:

defaults.device: iosv

module: [ ospf ]

nodes: [ r1, r2 ]
links:
- r1-r2
- r1:
  type: loopback
- r2:
  type: loopback

Even though netlab implements loopback links with loopback interfaces, they still behave as stub links. For example, the IP prefix assigned to a stub link comes from the LAN pool (like for the “real” stub links).

Having a granular configuration is nice, but it’s also tedious. That’s why you can set the default behavior in the system defaults:

  • defaults.links.stub_loopback parameter (boolean, True = use loopbacks) sets system-wide behavior
  • defaults.devices.<device>.features.stub_loopback parameter sets the behavior for an individual device type.

For example, if you want to test how Arista vEOS works, but need a few extra devices in your lab to advertise the subnets, set the **stub_loopback **parameter just for the extras:

defaults.device: iosv
defaults.devices.iosv.features.stub_loopback: True

module: [ ospf ]

nodes:
  rtr:
    device: eos
  x1:
  x2:

links:
- rtr-x1
- rtr-x2
- rtr
- x1
- x2
The default settings are not checked in the same way as topology attributes – you’ll get no error message if you make a typo (do I have to explain how I know that?)

Getting Started

To get more details and learn about additional features included in release 1.5.0, read the release notes. To upgrade, execute pip3 install --upgrade networklab.

New to netlab? Start with the Getting Started document and the installation guide.

Revision History

2023-03-06
Fixed the initial lab topology which incorrectly included type: loopback (thanks a million to Sander Steffann for reporting the typo).
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