Configuring SRv6

This document describes the device data model parameters one should consider when creating an SRv6 device configuration template. For a wider picture, please see the contributing new devices document.

The configuration template (in Jinja2 format) should be stored in the netsim/ansible/templates/srv6/ directory. The template file name is derived from the netlab_device_type or ansible_network_os variable (see Configuring BGP for details on how the platform name is used).

Tip

The <platform> is the value of the netlab_device_type or ansible_network_os variable.

Device Features

Add the following features.srv6 entries to a device YAML file (netsim/devices/<device>.yml) to indicate which SRv6 features the platform supports:

isis – device supports SRv6 with IS-IS
ospf – device supports SRv6 with OSPFv3
bgp – device supports BGP v4/v6 overlay over SRv6 (end.dt4/end.dt6)
vpn – device supports BGP L3VPN v4/v6 over SRv6

Node Attributes

The srv6 node dictionary is always present on nodes that have the srv6 module enabled. It contains the following attributes after transformation:

srv6.locator

An IPv6 prefix in CIDR notation (e.g., 5f00:0:1::/48), specified in lab topology or automatically allocated per-node from the SRv6 locator address pool (default 5F00::/16, based on the IANA-reserved range in RFC 9602).

Templates typically use srv6.locator to configure the SRv6 locator prefix:

locator {{ inventory_hostname }}
  prefix {{ srv6.locator }}

srv6.igp

A list of IGP protocol names (e.g., ['isis']) over which SRv6 should be signalled. The default is ['isis']. Valid values are isis and ospf.

Use if 'isis' in srv6.igp (or 'ospf' in srv6.igp) to conditionally enable SRv6 under an IGP:

{% if 'isis' in srv6.igp %}
router isis {{ isis.instance }}
  segment-routing srv6
    locator {{ inventory_hostname }}
{% endif %}

srv6.bgp

An optional dictionary with keys ipv4 and/or ipv6, each containing a list of BGP neighbor types that should use SRv6 as the transport for the plain IPv4 or IPv6 unicast address family (end.dt4/end.dt6, not VPN).

srv6.bgp is absent when the feature is disabled.

Example:

{% if srv6.bgp is defined %}
{%   for af in ['ipv4','ipv6'] if af in srv6.bgp %}
  family {{ af }}
    locator {{ inventory_hostname }}
{%   endfor %}
{% endif %}

srv6.vpn

A dictionary with keys ipv4 and/or ipv6, each containing a list of BGP neighbor types for which BGP L3VPN routes should be carried over SRv6 (end.dt4-vrf/end.dt6-vrf).

The structure and boolean-expansion rules are identical to srv6.bgp. srv6.vpn is absent when L3VPN is disabled.

srv6.transit_only

A boolean flag (True/False). When True, the node acts as a transit-only SRv6 node — it forwards SRv6 traffic but does not need to allocate FPE resources or end.dt SIDs for originating/terminating traffic. Default is False.

{% if not srv6.transit_only|default(False) %}
  termination-fpe: [2]
{% endif %}

Loopback IPv6 Address

loopback.ipv6 is always present on SRv6 nodes (enforced by node_post_transform). It may be user-defined, assigned from the loopback topology pool, or automatically assigned as the first host address in srv6.locator (when srv6.allocate_loopback: True).

Templates must use loopback.ipv6 as the SRv6 source address:

source-address {{ loopback.ipv6 | ansible.utils.ipaddr('address') }}

BGP Neighbor Attributes

The srv6 module modifies the bgp.neighbors list during node_post_transform. For each BGP neighbor that…

has an ipv6 transport address
has no ipv4 transport address,
is listed in srv6.bgp.ipv4 or srv6.vpn.ipv4 (i.e., IPv4 AF is carried over an IPv6 session)

…the module adds:

extended_nexthop (True) — signals that the BGP session requires extended next-hop encoding (RFC 8950) to carry IPv4 prefixes over an IPv6 transport session.

Templates could check for this attribute to conditionally enable extended next-hop negotiation:

{% for n in bgp.neighbors|default([]) if n.ipv6 is defined %}
{%   if n.extended_nexthop is defined %}
  neighbor {{ n.ipv6 }} capability extended-nexthop
{%   endif %}
{% endfor %}

Interface Attributes

The srv6 module does not add any per-interface attributes. Templates that need SRv6 interface-level configuration (e.g., end.x SIDs) typically iterate over interfaces that have IS-IS or OSPF enabled:

{% set srv6_interfaces = interfaces | selectattr('isis','defined') | list %}

Template Architecture

Complex platforms may split SRv6 configuration into a main template and a conditionally-included BGP sub-template. The main template handles locator declaration and IGP integration; the BGP sub-template handles the BGP overlay and VPN configuration.

Example (frr.j2 main template, simplified):

segment-routing
 srv6
  encapsulation
   source-address {{ loopback.ipv6 | ansible.utils.ipaddr('address') }}
  locators
   locator {{ inventory_hostname }}
    prefix {{ srv6.locator }}

{% if 'isis' in srv6.igp|default([]) and isis.instance is defined %}
router isis {{ isis.instance }}
 segment-routing srv6
   locator {{ inventory_hostname }}
{% endif %}

{% if srv6.vpn is defined and bgp.as is defined %}
{%   include "frr.bgp.j2" %}
{% endif %}

Example (frr.bgp.j2 BGP sub-template, simplified):

router bgp {{ bgp.as }}
 segment-routing srv6
  locator {{ inventory_hostname }}

{% if srv6.vpn is defined %}
{%   for n in bgp.neighbors|default([]) if n.ipv6 is defined %}
{%     if n.extended_nexthop is defined %}
 neighbor {{ n.ipv6 }} capability extended-nexthop
{%     endif %}
{%     for af in ['ipv4','ipv6'] if n.type in srv6.vpn.get(af,[]) %}
 address-family {{ af }} vpn
  neighbor {{ n.ipv6 }} activate
{%     endfor %}
{%   endfor %}
{% endif %}

Example (sros.j2, Nokia SR-OS — gNMI/YANG path style):

{% set srv6_interfaces = interfaces | selectattr('isis','defined') | list %}
updates:
- path: configure/router[router-name=Base]/segment-routing/segment-routing-v6
  val:
   locator:
   - locator-name: JvB
     prefix:
      ip-prefix: {{ srv6.locator }}
{%   if not srv6.transit_only|default(False) %}
     termination-fpe: [2]
{%   endif %}

{% if 'isis' in srv6.igp %}
- path: configure/router[router-name=Base]/isis[isis-instance=0]
  val:
   segment-routing-v6:
    admin-state: enable
    locator:
    - locator-name: JvB
{% endif %}

Integration Tests

The integration tests for the SRv6 module are in the tests/integration/srv6/ directory:

02-isis-ipv4-bgp-vpn.yml — PE router running IS-IS + BGP L3VPN (srv6.vpn.ipv4: True) over an IPv6-only SRv6 core, with VRFs attached; validates end-to-end L3VPN reachability.
12-isis-ipv6-bgp-vpn.yml — Similar scenario with IPv6 L3VPN overlay.
x-01-isis-ipv4-bgp.yml — PE router running IS-IS + BGP (srv6.bgp.ipv4: True) with allocate_loopback: True, carrying IPv4 over an IPv6-only SRv6 core (4PE scenario).
x-11-isis-ipv6-bgp.yml — Similar scenario with IPv6 BGP overlay.