MPLS LDP Configuration Templates

This document describes how to create or modify MPLS LDP configuration templates for network devices supported by netlab.

MPLS LDP (Label Distribution Protocol) is a sub-feature of the MPLS module. It is enabled alongside other MPLS features (BGP-LU, VPN, 6PE) but has its own configuration template.

Enabling LDP Support on a Device

To enable LDP support on a device, add the mpls.ldp feature flag to the device YAML file (netsim/devices/<device>.yml):

mpls:
  ldp: true

The LDP configuration template should be stored in the netsim/ansible/templates/mpls/ directory (name <platform>.ldp.j2) or within the netsim/ansible/templates/mpls/<platform> directory (name: ldp.j2).

Template Variables

The following variables are available in LDP configuration templates:

Node-Level Variables

Variable	Type	Description
`ldp.router_id`	string	LDP router ID (IPv4 address)
`ldp.explicit_null`	bool	Enable explicit null labels (default: False)
`ldp.af.ipv4`	bool	Enable LDP
`ldp.af.ipv6`	bool	Enable LDPv6 (optional, not tested)

Interface Variables

When iterating over interfaces, use this pattern to find LDP-enabled interfaces:

{% for l in interfaces if ('ldp' in l) and not l.ldp.passive %}
interface {{ l.ifname }}
  mpls ip
{% endfor %}

Variable	Description
`l.ifname`	Interface name
`l.ldp.passive`	LDP is not enabled on this interface

Tip

There is no ‌ldp.passive interface attribute. The ‌ldp.passive attribute is set internally on stub links and links with ‌role set to ‌passive. For LDP, ‌passive is functionally identical to ‌do not run LDP on this interface.

Template Examples

Arista EOS

!
mpls ip
mpls icmp ttl-exceeded tunneling
!
mpls ldp
  router-id {{ ldp.router_id }}
  transport-address interface Loopback0
  interface disabled default
  no shutdown
{% if ldp.explicit_null|default(False) %}
  label local-termination explicit-null
{% endif %}
!
interface Loopback0
  mpls ldp interface
!
{% for l in interfaces if ('ldp' in l) and not l.ldp.passive %}
interface {{ l.ifname }}
  mpls ip
  mpls ldp interface
{% endfor %}

Key points:

Global mpls ip enables MPLS forwarding
LDP router ID and transport address (usually loopback)
Enable LDP on loopback interface
Enable LDP on all relevant interfaces (interface must have ldp dictionary, but should not have ldp.passive set to True)

Cisco IOS

!
mpls ldp router-id Loopback0
mpls ldp logging neighbor-changes
{% if ldp.explicit_null|default(False) %}
mpls ldp explicit-null
{% endif %}
!
{% for l in interfaces if ('ldp' in l) and not l.ldp.passive %}
interface {{ l.ifname }}
 mpls ip
{% endfor %}
!

FRRouting

!
mpls ldp
  router-id {{ ldp.router_id }}
{% if not 'ipv6' in loopback %}
  dual-stack transport-connection prefer ipv4
{% endif %}
{% for af in ['ipv4','ipv6'] if ldp.af[af] is defined %}
  address-family {{ af }}
{%   if loopback[af] is defined %}
    discovery transport-address {{ loopback[af]|ansible.utils.ipaddr('address') }}
{%   endif %}
{%   if ldp.explicit_null|default(False) %}
    label local advertise explicit-null
{%   endif %}
{%   for l in interfaces if ('ldp' in l) and (af in l) and not l.ldp.passive %}
    interface {{ l.ifname }}
    exit
{%   endfor %}
  exit
{% endfor %}
!

FRR supports:

IPv4 and IPv6 address families
Dual-stack transport connections
Per-AF transport addresses

VRF/LDP CSC Support (WIP)

LDP can be used within VRFs (LDP Carrier Supporting Carrier - CsC). This requires:

Setting mpls.ldp: true on individual VRF interfaces
Device support for MPLS CsC (checked via device features)

Testing

Use these integration tests (in tests/integration/mpls directory) when adding LDP support to a new platform:

01-ldp-p – LDP functionality on P-device (establishing LDP adjacency, exchanging labels, building working LSPs)
02-ldp-pe – LDP functionality on PE-device (use LDP to assign labels to BGP next hops)