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Linux bridge#

Containerlab can connect its nodes to a Linux bridge instead of interconnecting the nodes directly. This connectivity option is enabled with bridge kind and opens a variety of integrations that containerlab labs can have with workloads of other types.

For example, by connecting a lab node to a bridge we can:

  1. allow a node to talk to any workload (VM, container, baremetal) which are connected to that bridge
  2. let a node to reach networks which are available via that bridge
  3. scale out containerlab labs by running separate labs in different hosts and get network reachability between them
  4. wiring nodes' data interfaces via a broadcast domain (linux bridge) and use vlans to making dynamic connections

Using bridge kind#

Containerlab doesn't create bridges on users behalf, that means that in order to use a bridge in the topology definition file, the bridge needs to be created and enabled first.

Once the bridge is created, it needs to be referenced as a node inside the topology file:

# topology documentation:
name: br01

      type: ixrd2l
      kind: nokia_srlinux
      kind: nokia_srlinux
      kind: nokia_srlinux
    # note, that the bridge br-clab must be created manually
      kind: bridge

    - endpoints: ["srl1:e1-1", "br-clab:eth1"]
    - endpoints: ["srl2:e1-1", "br-clab:eth2"]
    - endpoints: ["srl3:e1-1", "br-clab:eth3"]

In the example above, node br-clab of kind bridge tells containerlab to identify it as a linux bridge and look for a bridge named br-clab.

When connecting other nodes to a bridge, the bridge endpoint must be present in the links section.


When choosing names of the interfaces that need to be connected to the bridge make sure that these names are not clashing with existing interfaces.
In the example above we named interfaces eth1, eth2, eth3 accordingly and ensured that none of these interfaces existed before in the root netns.

As a result of such topology definition, you will see bridge br-clab with three interfaces attached to it:

bridge name     bridge id               STP enabled     interfaces
br-clab         8000.6281eb7133d2       no              eth1

Containerlab automatically adds an iptables rule for the referenced bridges to allow forwarding over them. Namely, for a given bridge named br-clab containerlab will attempt to call the following iptables command during the lab deployment:

iptables -I FORWARD -i br-clab -j ACCEPT

This will ensure that traffic is forwarded when passing this particular bridge. Note, that once you destroy the lab, the rule will stay.

Check out "External bridge" lab for a ready-made example on how to use bridges.