BGP-LS Session

BGP-LS (BGP Link-State, RFC 7752) lets a router export its OSPF or IS-IS link-state database into BGP. In BGP-LS mode, a Watcher receives those updates and feeds the topology into Topolograph without any GRE tunnel or IGP adjacency — which makes it the easiest live method to deploy, especially across multiple areas or IS-IS levels.

Minimum versions

BGP-LS ingestion is supported from Docker image vadims06/ospf-watcher:v3.1.0 (and the equivalent IS-IS Watcher image). Older images are GRE-only.

How it works

flowchart LR
    R[Router<br/>OSPF / IS-IS] -->|BGP-LS update<br/>AFI/SAFI Link-State| GB[GoBGP<br/>bgplswatcher]
    GB -->|gRPC| W[Watcher<br/>OSPF / IS-IS]
    W -->|topology + events| T[Topolograph]
    W --> ELK[ELK / Zabbix / Slack]

1. The router is configured to advertise its OSPF/IS-IS topology over BGP-LS.
2. GoBGP — packaged as the bgplswatcher component — establishes the BGP session and receives the Link-State address-family updates.
3. bgplswatcher forwards those updates to the Watcher over gRPC.
4. The Watcher processes them and posts the topology (and change events) to Topolograph.

Because there's no GRE tunnel and no OSPF/IS-IS adjacency to maintain, BGP-LS mode is simpler to deploy in environments where tunnels aren't practical, and a single session can carry topology from across the IGP domain.

Why the BGP-LS Watcher?

GoBGP handles the BGP machinery and speaks the Link State address family. The BGP-LS Watcher (Go) bridges GoBGP to the Python Watcher over gRPC and keeps GRE and BGP-LS modes behaving identically.

1. Configure BGP-LS on the router

Enable the BGP Link State address family and configure BGP to distribute the IGP's link-state information (activate BGP-LS). The exact commands are vendor-specific — consult your vendor's documentation for the BGP-LS address family configuration. Point the BGP-LS peer at the Watcher host so GoBGP can receive the updates.

2. Deploy the Watcher in BGP-LS mode

Use the BGP-LS variant of the Watcher, which brings up the bgplswatcher (GoBGP) container alongside the Watcher. Setup details and compose files are in the Watcher repositories: OSPF Watcher · IS-IS Watcher.

3. Verify the BGP-LS session

The Watcher posts topology to Topolograph after the BGP session comes up, so start by confirming the session and the Link-State routes.

Check the bgplswatcher container logs:

docker logs watcher<num>-bgpls-ospf-bgplswatcher

Inspect the session with the bundled gobgp CLI:

# List BGP neighbors and session state
docker exec -it watcher<num>-bgpls-ospf-bgplswatcher gobgp neighbor

# Detailed status for one neighbor
docker exec -it watcher<num>-bgpls-ospf-bgplswatcher gobgp neighbor <neighbor-ip>

# Link-State routes received from a neighbor
docker exec -it watcher<num>-bgpls-ospf-bgplswatcher gobgp neighbor <neighbor-ip> adj-in -a ls

# Everything in the Link-State RIB
docker exec -it watcher<num>-bgpls-ospf-bgplswatcher gobgp global rib -a ls

Once the neighbor shows Established and Link-State routes appear in the RIB, the Watcher will start posting topology to Topolograph.

Traffic Engineering over BGP-LS

BGP-LS carries TE attributes natively — administrative group/color, maximum and reservable bandwidth, unreserved bandwidth, and the TE default metric — so you get rich link data with TE attributes by default. See Traffic Engineering.

BGP-LS vs GRE

	GRE	BGP-LS
Tunnel required	✅ GRE	❌
IGP adjacency	✅ (FRR over GRE)	❌
Router feature	GRE + OSPF/IS-IS	BGP-LS export
Scales across areas/levels	per attachment point	single session
Min. Watcher image	any	v3.1.0+

Compare with GRE

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Next: see what the Watcher does with the feed → OSPF Watcher · IS-IS Watcher