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Topology Components

Large lab topologies tend to be structured. For example, you might have two sites with a leaf-and-spine fabric, or a leaf-and-spine fabric with multiple racks.

Topology components allow you to define reusable smaller structures that you can include multiple times into the same lab topology.

Defining Components

Topology components are defined in the components dictionary. Each value in that dictionary should have nodes element (nodes within the component), links list (intra-component links) and any other attribute that is a valid node-level attribute, for example BGP AS number.

Imagine you have a lab topology with multiple sites, and each site has a PE-router and a P-router. The site structure could be defined as a component:

  • It has two nodes (P and PE)

  • P-router runs OSPF and MPLS

  • PE-router runs OSPF, BGP, and MPLS

  • The default OSPF cost for intra-site links is 100

  • The default BGP AS number is 65000

        module: [ ospf, mpls ]
        module: [ ospf, bgp, mpls ]
    links: [ p-pe ]
    ospf.cost: 100 65000

Using Components

Use include node attribute to include a component into the topology. You can add any node-level attribute you wish to the node with the include attribute; such attributes will be copied to the nodes created from the included component.

For example, use the following nodes dictionary to build a lab topology with three sites (each of them defined as a site component), and set different default OSPF cost for the third site (S3):

    include: site
    include: site
    include: site
    ospf.cost: 200

Including Components

Whenever you include a component into a lab topology, netlab creates new nodes, links, and a component-specific group that includes component- and parent node settings. The node names of the new nodes are composed from the parent node name and the component node name.


It’s easy to exceed the maximum node name length (16 characters) when using lab components. To increase the maximum node name length, set the defaults.const.MAX_NODE_ID_LENGTH lab topology attribute (see also: changing defaults).

The above three-site lab topology would result in:

  • Six nodes: s1_p, s1_pe, s2_p, s2_pe, s3_p, and s3_pe

  • Three links: s1_p-s1_pe, s2_p-s2_pe, and s3_p-s3_pe

  • Three groups: inc_s1, inc_s2 and inc_s3

The inc_s3 group would have two members (s3_p and s3_pe) and two node parameters: ospf.cost set to 200 (copied from the s3 node) and set to 65000 (copied from the site component).

As the final stage of a component expansion, the parent node is removed from the list of nodes.