VLANs

The VLAN configuration module implements VLANs and VLAN-related interfaces. The current initial implementation supports:

  • Access VLANs

  • VLAN trunks, including configurable native VLAN

  • VLAN interfaces (integrated routing and bridging)

  • Bridging-only and IRB VLANs

The following features are currently on the radar:

  • Routed subinterfaces

Warning

This module is experimental for a reason. The VLAN transformation code is the biggest bowl of spaghetti code in the whole system, and is probably full of bugs. We’ll whack them as we create more interesting scenarios, but it will take us a while to get there.

Platform Support

VLANs are supported on these platforms:

Operating system Access
VLANs
VLAN
interfaces
Routed
subinterfaces
Trunk
ports
Native
VLAN
Arista EOS
Cisco IOSv
VyOS
Dell OS10

VLAN Connectivity Model

The VLAN configuration module assumes you’re creating a sane design in which:

  • VLAN numbers are globally unique (you’re not reusing 802.1q values)

  • Every VLAN is contiguous and might span multiple physical links (please note that VLANs bridged across VXLAN or MPLS are still contiguous)

  • Every VLAN uses a unique IP subnet across all physical links where it’s used.

  • On access links, all VLAN-capable devices connected to a link use the same access VLAN.

  • On trunk links, all VLAN-capable devices using native VLAN use the same native VLAN.

It might be possible to build topologies that deviate from these rules, but don’t be surprised when the results look weird.

Parameters

The following parameters can be set globally or per node:

  • vlans: A dictionary of VLAN definitions (see below)

  • vlan.mode: The default VLAN forwarding mode (bridge or irb).

The following global parameters are used to set VLAN IDs and VNIs in VLAN definitions:

  • vlan.start_vlan_id: This global value specifies the first auto-assigned VLAN ID (default: 1000).

  • vlan.start_vni: This global value specifies the first auto-assigned VNI (default: 100000).

VLAN Definition

VLANs are defined in a global- or node-specific vlans dictionary, allowing you to create network-wide VLANs or local VLANs.

Warning

Use unique VLAN names when defining node-specific VLANs. There’s a subtle interaction between global- and node-specific VLAN definitions.

The keys of the vlans dictionary are VLAN names, the values are VLAN definitions. A VLAN definition could be empty or a dictionary with one or more of these attributes:

  • id – 802.1q VLAN tag

  • vni – VXLAN VNI

  • vrf – the VRF VLAN belongs to

  • prefix – IP prefix assigned to the VLAN. The value of the prefix could be an IPv4 prefix or a dictionary with ipv4 and ipv6 keys.

  • pool – addressing pool used to assign IPv4/IPv6 prefixes to the VLAN. VLAN prefixes are allocated from addressing pools before interface address assignments.

  • A VLAN definition can also contain other valid interface-level parameters (for example, OSPF cost).

VLAN definitions lacking id or vni attribute get default VLAN ID and VNI values assigned during the topology transformation process.

Default VLAN Values

VLAN definitions without id or vni attribute will get a VLAN ID or VNI assigned automatically. The first auto-assigned VLAN ID is specified in the vlan.start_id global attribute; ID assignment process skips IDs assigned to existing VLANs.

Creating VLAN Interfaces and Routed Subinterfaces

VLAN interfaces are created on-demand based on these rules:

  • A VLAN/SVI/BVI interface is created for every VLAN with mode set to bridge or irb present on a node.

  • VLAN subinterfaces are created on VLAN trunks on platforms behaving more like routers than switches (example: Cisco IOS).

The VLAN mode can be set in global- or node vlans dictionary or with the mode attribute of interface/link vlan dictionary or trunk dictionary within vlan dictionary.

The default VLAN mode is specified in global or node vlan.mode attribute.

VLAN Interface Parameters

You can set link- or interface-level parameters within the node vlans dictionary to change VLAN interface parameters.

For example, use the following definitions to set the OSPF cost for the red VLAN interface on node s1:

vlans:
  red:

nodes:
  s1:
    module: [ ospf,vlan ]
    vlans:
      red:
        ospf.cost: 10

links:
- s1:
    vlan.access: red
  ...

You can also set interface parameters for every VLAN interface connected to a VLAN within global VLAN definition. For example, you could set the OSPF cost for all VLAN interfaces connected to the red VLAN:

vlans:
  red:
    ospf.cost: 10

nodes:
  s1:
    module: [ ospf,vlan ]

links:
- s1:
    vlan.access: red
  ...

Warning

Don’t try to set VLAN interface parameters on access or trunk links; you might get unexpected results.

Physical Interface and VLAN Interface Addressing

IPv4 and/or IPv6 prefixes are automatically assigned to VLAN-enabled links:

  • A VLAN is treated like a multi-access link with an IPv4/IPv6 prefix, and gets an addressing prefix assigned from the corresponding address pool if needed.

  • All nodes connected to a VLAN get their IP addresses from the VLAN prefix (see below). Node addresses within a VLAN prefix are calculated with the algorithm used to calculate IP addresses on multi-access links.

  • Whenever a VLAN access interface is attached to a link, the VLAN prefix is used to assign IP addresses to all nodes on that link.

  • A VLAN trunk is decomposed into a number of virtual links (one per VLAN). The above rules are then applied to those virtual links.

The following rules are used to assign VLAN IPv4/IPv6 addresses to node interfaces:

  • When a node is attached to a VLAN-enabled link, but does not have a vlan interface attribute, the VLAN IP address is assigned to physical interface.

  • When the VLAN forwarding mode is set to irb, the node VLAN IP address is assigned to a VLAN interface.

  • No IP address is assigned to the VLAN interface when the VLAN forwarding mode is set to bridge.

  • No IP address is assigned to the physical interface that has an access or native VLAN. You can force an IP address assignment to such an interface with ipv4 or ipv6 interface attribute and become responsible for the results of your actions.

  • IP prefixes are not assigned to the physical interfaces with VLAN trunks. If you want to assign IP addresses to default native VLAN (1), use role or prefix link attribute.