.. _bgp-link-state:
BGP Link-State
==============
BGP Link-State (BGP-LS) is a mechanism to distribute network topology information
via BGP. It enables BGP to carry link-state information learned from IGP protocols
(IS-IS, OSPF) to other BGP speakers. This is useful for applications such as
Software-Defined Networking (SDN) controllers, Traffic Engineering, and network
visualization tools that need a complete view of the network topology.
BGP-LS is defined in :rfc:`9552`.
Overview
--------
BGP-LS introduces a new BGP AFI/SAFI combination (AFI 16388, SAFI 71) to carry
link-state Network Layer Reachability Information (NLRI). The link-state NLRI
describes nodes, links, and prefixes in the network topology.
Three types of BGP-LS NLRI are defined:
- **Node NLRI** - Describes routers/nodes in the network
- **Link NLRI** - Describes links between nodes
- **Prefix NLRI** - Describes IPv4/IPv6 prefixes advertised by nodes
BGP-LS carries topology information in a new BGP path attribute called the
BGP-LS Attribute (type 29), which contains various TLVs describing node
properties, link metrics, bandwidth, prefix attributes, and other topology
characteristics.
Configuration
-------------
BGP-LS Producer Configuration
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
To enable BGP-LS and originate topology information from the local IGP, configure
the link-state address family:
.. code-block:: frr
router bgp 65001
neighbor 192.0.2.1 remote-as 65002
!
address-family link-state
neighbor 192.0.2.1 activate
exit-address-family
When the link-state address family is activated on a neighbor, BGP will:
1. Negotiate BGP-LS capability during session establishment
2. Receive IGP topology updates from Zebra (IS-IS and OSPF)
3. Convert IGP topology to BGP-LS NLRI (Node, Link, Prefix)
4. Advertise BGP-LS routes to the activated neighbor
BGP-LS Consumer Configuration
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
To receive and process BGP-LS information from peers without originating routes:
.. code-block:: frr
router bgp 65002
neighbor 192.0.2.2 remote-as 65001
!
address-family link-state
neighbor 192.0.2.2 activate
exit-address-family
BGP-only Fabric Topology Distribution
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
In a BGP-only fabric, topology is derived from BGP sessions and BGP routing
state rather than from IGP protocols.
This behavior aligns with the IETF document
`draft-ietf-idr-bgp-ls-bgp-only-fabric
`_.
FRR uses BGP-LS to advertise Node, Link, and Prefix topology for the fabric.
Use the following command under the link-state address family to enable
export of BGP-only fabric topology:
.. clicmd:: distribute bgp-fabric-link-state [instance-id WORD]
Enable BGP-LS export for BGP-only fabrics. When enabled, FRR originates
BGP-LS Node, Link, and Prefix NLRIs from BGP adjacency and BGP RIB data.
This command is configured under ``address-family link-state link-state``.
``instance-id`` is optional and sets the BGP-LS identifier in originated
NLRIs. If omitted, FRR uses ``0``.
Example for a BGP-only fabric speaker with default instance-id:
.. code-block:: frr
router bgp 65001
neighbor 10.255.1.2 remote-as 65000
!
address-family link-state link-state
distribute bgp-fabric-link-state
neighbor 10.255.1.2 activate
exit-address-family
Example for a BGP-only fabric speaker with explicit instance-id:
.. code-block:: frr
router bgp 65001
neighbor 10.255.1.2 remote-as 65000
!
address-family link-state link-state
distribute bgp-fabric-link-state instance-id 100
neighbor 10.255.1.2 activate
exit-address-family
Per-neighbor Link Identifier Configuration
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The following commands let you override local and remote link identifiers
carried in BGP-LS Link NLRIs for a specific BGP neighbor. These commands are
configured under ``router bgp`` and accept 32-bit values.
.. clicmd:: neighbor local-link-id (1-4294967295)
Set the local link identifier in the neighbor's Link NLRI.
If unset, FRR uses the interface kernel ifindex (a unique per-interface
identifier) as the fallback value.
.. clicmd:: neighbor remote-link-id (1-4294967295)
Set the remote link identifier in the neighbor's Link NLRI.
If unset, FRR uses ``0`` as fallback.
Displaying BGP-LS Information
------------------------------
Show All BGP-LS NLRIs
^^^^^^^^^^^^^^^^^^^^^^^
.. clicmd:: show bgp link-state link-state [json]
Display all BGP-LS routes in the link-state RIB. Shows Node, Link, and Prefix
NLRI with their associated attributes.
Example output:
.. code-block:: frr
router# show bgp link-state link-state
BGP table version is 15, local router ID is 192.0.2.1
Status codes: s suppressed, d damped, h history, * valid, > best, = multipath,
i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> [1][0.0.0.0][6][0][0][0300.0000.0001]
0.0.0.0(vrf1) 0 32768 ?
*> [2][0.0.0.0][6][0][0][0300.0000.0001][0300.0000.0002][192.0.2.1][192.0.2.2]
0.0.0.0(vrf1) 0 32768 ?
With the ``json`` option, output is displayed in JSON format suitable for
programmatic processing.
Show Specific BGP-LS NLRI
^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. clicmd:: show bgp link-state link-state NLRI [json]
Display detailed information about a specific BGP-LS NLRI identified by its
NLRI string. The NLRI string format is:
- Node NLRI: ``[1][IGP-ID][Protocol][Instance][AS][Area/Domain][IGP-Router-ID]``
- Link NLRI: ``[2][IGP-ID][Protocol][Instance][AS][Area][Local-Router-ID][Remote-Router-ID][Local-IP][Remote-IP]``
- Prefix NLRI: ``[3|4][IGP-ID][Protocol][Instance][AS][Area][Router-ID][Prefix]``
Example:
.. code-block:: frr
router# show bgp link-state link-state [1][0.0.0.0][6][0][0][0300.0000.0001]
BGP routing table entry for [1][0.0.0.0][6][0][0][0300.0000.0001]
Paths: (1 available, best #1, table link-state)
Not advertised to any peer
Local from 0.0.0.0 (192.0.2.1)
Origin incomplete, metric 0, localpref 100, valid, sourced, local, best (First path received)
Last update: Thu Jan 23 10:15:42 2026
Node Descriptors:
Protocol-ID: IS-IS Level 2
AS Number: 0
IGP Router-ID: 0300.0000.0001
Node Attributes:
Router-ID: 192.0.2.1
ISIS Area-ID: 49.0001
Node Flags: 0x00
Use ``show bgp [afi] [safi] summary`` to display BGP session summary including
link-state address family statistics.
Debugging
---------
.. clicmd:: debug bgp linkstate
Enable debugging for BGP-LS operations including:
- NLRI encoding/decoding
- Attribute processing
- IGP topology updates
- Route origination and withdrawal
- TED updates
Configuration Example
---------------------
Complete example with IS-IS IGP and BGP-LS:
.. code-block:: frr
! IS-IS configuration
router isis 1
net 49.0001.0300.0000.0001.00
is-type level-2-only
topology ipv6-unicast
!
interface eth0
ip router isis 1
isis network point-to-point
!
! BGP configuration with Link-State
router bgp 65001
bgp router-id 192.0.2.1
neighbor 192.0.2.100 remote-as 65002
neighbor 192.0.2.100 description SDN-Controller
!
address-family link-state
neighbor 192.0.2.100 activate
exit-address-family
In this example:
- IS-IS is configured to run on the local router
- BGP session is established with an SDN controller (192.0.2.100)
- Link-state address family is activated for the controller neighbor
- BGP-LS will automatically convert IS-IS topology to BGP-LS NLRI and
advertise it to the controller
Use Cases
---------
BGP-LS is typically used in the following scenarios:
**SDN Controllers**
SDN controllers use BGP-LS to obtain a complete view of the network topology
for path computation, traffic engineering, and network optimization.
**PCE (Path Computation Element)**
PCE servers use BGP-LS to maintain an up-to-date TED for computing MPLS-TE
and SR-TE paths.
**Network Monitoring and Visualization**
Network management systems use BGP-LS to visualize network topology and
monitor link utilization, metrics, and failures.
**Multi-Domain TE**
BGP-LS enables traffic engineering across multiple IGP domains by sharing
topology information between domains via BGP.