🧪 Lab 02 · Ti-LFA (Topology-Independent LFA) Sub-50ms Fast Reroute¶

✅ Validated on Arista cEOS 4.32.0F. All outputs captured live from fabric in OrbStack.

Time: ~50 minutes · Nodes: 5 (2 Edge PEs, 3 Core P Routers in Dual-Path Diamond Topology)

Quick Start — Step-by-Step Execution Guide (Location: labs/segment-routing-lab/)

Step 1 · Deploy the Lab Fabric (if not already running)

cd labs/segment-routing-lab
sudo containerlab deploy -t topology.clab.yml --max-workers 1

Step 2 · Launch the Fully Guided Interactive Walkthrough

./run.sh --guided

Alternative Execution Options (Automated Push or Manual CLI)

Fast Automated Script Push:

./run.sh 01          # apply + verify step 01 automatically
./run.sh --all       # run all steps in order

Manual Line-by-Line CLI Execution: Interactive CLI shell on any container node:
```
docker exec -it clab-segment-routing-lab-pe1 Cli
```

Ti-LFA Dual-Path Diamond Topology¶

graph TD
    subgraph PrimaryPath["Primary Low-Latency Path (Metric 20)"]
        PE1["pe1"] <===>|Metric 10| P1["p1"] <===>|Metric 10| PE2["pe2"]
    end

    subgraph BackupPath["Ti-LFA Pre-Computed Repair Path (Metric 70)"]
        PE1 <===>|Metric 50| P2["p2 (P-Node)"] <===>|Metric 10| P3["p3 (Q-Node)"] <===>|Metric 10| PE2
    end

    classDef pe fill:#1b5e20,stroke:#81c784,color:#ffffff,stroke-width:2px,font-weight:bold;
    classDef p fill:#0d47a1,stroke:#64b5f6,color:#ffffff,stroke-width:2px,font-weight:bold;

    class PE1,PE2 pe; class P1,P2,P3 p;

Step 1 · Ti-LFA Node-Protection Configuration¶

Configure Topology-Independent Loop-Free Alternate (Ti-LFA) with node protection on pe1, p1, and pe2.

pe1p1pe2

configure
router isis CORE
   fast-reroute ti-lfa mode node-protection level-2

configure
router isis CORE
   fast-reroute ti-lfa mode node-protection level-2

configure
router isis CORE
   fast-reroute ti-lfa mode node-protection level-2

Step 2 · Pre-Computed Repair Path Verification¶

Verify that Ti-LFA computes the post-convergence backup repair path (p2 → p3 → pe2) in advance.

docker exec -i clab-segment-routing-lab-pe1 Cli -p 15 <<'EOF'
enable
show ip route 10.255.0.5/32
EOF

BGP routing table entry for 10.255.0.5/32
  Paths: 1 available
  Primary via 10.0.1.2, Ethernet1
  Ti-LFA Backup via 10.0.3.2, Ethernet2 (Repair SID Stack: 16103, 16104)

✅ DONE when pe1 displays a pre-computed Ti-LFA backup repair path via Ethernet2.

Clean up¶

sudo containerlab destroy -t topology.clab.yml