Unnumbered Interfaces

netlab supports unnumbered IPv4, IPv6 (LLA) or dual-stack interfaces. There are two ways to create an unnumbered interface (or link):

  • Set unnumbered: true on interface[1] or link.

  • Set ipv4 and/or ipv6 attribute to True.

unnumbered attribute is translated into ipv4 and/or ipv6 attributes set to True based on address families configured on node’s loopback interface. With the default addressing setup, unnumbered: True results in ipv4: True, to enable dual-stack unnumbered interfaces with unnumbered attribute, add ipv6 prefix to the loopback addressing pool. For more details, see unnumbered interfaces part of addressing document.

Implementing unnumbered interfaces

There is no standard way of implementing IPv4 unnumbered interfaces, and they might not be available on all platforms. IPv4 implementations of unnumbered interfaces usually use the loopback IPv4 address as the interface IPv4 address. For more details, read the unnumbered interfaces series of blog posts on ipSpace.net.

Unnumbered IPv6 interfaces should use link-local addresses, a standard IPv6 feature.

If your device supports IPv6 LLA-only interface, set topology-defaults.yml attribute devices.name.features.initial.ipv6.lla to True.

If your device supports IPv4 unnumbered interfaces, set topology-defaults.yml attribute devices.name.features.initial.ipv4.unnumbered to True.

Integration with IGP routing protocols

OSPF and IS-IS implementations might support unnumbered IPv4 interfaces[2]. The routing protocol configuration modules detect unnumbered IPv4 interfaces by checking the unnumbered and ipv4 attributes – if either one of them is set to True, the interface is an unnumbered IPv4 interface.

OSPFv2 can use unnumbered IPv4 interfaces on point-to-point links. If your device supports this functionality, set topology-defaults.yml attribute devices.name.features.ospf.unnumbered to True. OSPFv2 cannot run over multi-access unnumbered IPv4 links.

Some IS-IS implementations support unnumbered IPv4 P2P links. If your device supports this, set topology-defaults.yml attribute devices.name.features.isis.unnumbered.ipv4 to True.

Fewer IS-IS implementations support unnumbered multi-access IPv4 links. To indicate your device can do that, set topology-defaults.yml attribute devices.name.features.isis.unnumbered.network to True.


If you’re unsure what your device can do, set all three feature flags to True, start a lab, and check whether the adjacency- and routing tables are populated as expected.

Unnumbered EBGP sessions

Several vendors implemented EBGP sessions between well-known IPv6 LLA addresses[3]. netlab does not support this half-baked attempt and implements IPv6 LLA sessions only for those devices that can configure EBGP session on an interface.

Devices supporting interface-level EBGP sessions between auto-generated IPv6 LLA can use these sessions to:

  • Transport IPv6 prefixes with LLA next hop over IPv6 AF

  • Transport IPv4 prefixes with IPv6 LLA next hop according to RFC 8950.

netlab core and BGP configuration module do not support:

  • Running IPv4 AF with IPv4 next hops over IPv6 transport session

  • Running IPv4 AF with RFC 8950-style IPv6 next hops over numbered IPv6 interfaces or over IBGP sessions.

  • Creating IBGP sessions between IPv6 LLA addresses


You can always extend netlab functionality with plugins and custom configuration modules.

netlab will create an IPv6 LLA EBGP session whenever it finds a pair of devices connected to the same link if the devices:

  • Belong to different autonomous systems

  • Have ipv6 interface attribute set to True.

Whenever netlab encounters an EBGP session between IPv6 LLA interfaces, it sets local_if attribute in the neighbor data structure to simplify the device configuration templates.

If your device supports EBGP sessions between auto-generated IPv6 link-local addresses, set topology-defaults.yml attribute devices.name.features.bgp.ipv6_lla to True.

netlab device configuration templates will enable RFC 8950-style IPv4-over-IPv6 address family on IPv6 LLA sessions if the interface has ipv6 interface attribute set to True (indicating IPv6 LLA EBGP session) AND ipv4 interface attribute set to True.

RFC 8950-style IPv4 address family REALLY SHOULD NOT be enabled for:

  • EBGP sessions running on numbered IPv6 interfaces

  • Interfaces with IPv4 addresses regardless of the state of ipv6 attribute.

BGP configuration module simplifies the device configuration templates conforming to the above restriction with the ipv4_rfc8950 neighbor attribute which is set:

  • when link or interface unnumbered attribute is set to True on both EBGP neighbors or

  • when both IPv4 and IPv6 interface attributes are set to True on both EBGP neighbors.

If your device supports RFC 8950 (IPv4 with IPv6 next hops) on EBGP sessions between auto-generated IPv6 link-local addresses, set topology-defaults.yml attribute devices.name.features.bgp.rfc8950 to True.