Fixing Cisco SD-WAN Blackholes with OMP Send-Backup-Paths

 Fixing Cisco SD-WAN Blackholes with OMP Send-Backup-Paths

The Problem: SD-WAN Blackholes During Link Failures

In Cisco SD-WAN, the Overlay Management Protocol (OMP) is responsible for advertising routes between vEdges. However, by default, vSmart only advertises the best routes based on the OMP best-path selection algorithm.

Let’s take a real-world example:

📍 Scenario:

• vEdge-1 has two Transport Locators (TLOCs): MPLS (T11) and Biz-Internet (T12).
• When the MPLS TLOC on vEdge-1 fails, it stops advertising the OMP route to 10.1.1.0/24.
• vSmart selects the best available route, which still goes via vEdge-1 but over Biz-Internet.
• Problem? vEdge-3 doesn’t have an overlay tunnel to the Biz-Internet TLOC of vEdge-1.

🔻 Result:

• The route to 10.1.1.0/24 becomes invalid and unresolved.
• vEdge-3 completely loses connectivity to the data center despite having a valid tunnel to vEdge-2.
• Traceroute and ping fail to reach 10.1.1.1.

The Solution: Enabling OMP Send-Backup-Paths

By default, vSmart acts like a BGP Route Reflector, advertising only the best route. However, in SD-WAN environments where full IP reachability between TLOCs is not guaranteed, this behavior can create routing blackholes.

🔧 Fix: Enable Send-Backup-Paths on vSmart to ensure it also advertises the first set of non-best routes.

Configuration on vSmart:

vSmart(config)# omp send-backup-paths  
vSmart(config-omp)# commit  


📈 What Changes?
✅ vSmart now advertises routes via vEdge-2, alongside the best route via vEdge-1.
✅ vEdge-3 learns an alternative path to 10.1.1.0/24 via vEdge-2.
✅ IP reachability is restored, and remote sites can reach the data center again.
Validating the Fix:
1️⃣ Check OMP Routes on vEdge-3:

show omp routes 10.1.1.0/24  
Now, we see both primary and backup routes advertised.
2️⃣ Run a Traceroute:


traceroute 10.1.1.1  
Traffic now flows via vEdge-2 → Data Center.
Key Takeaways
🔹 vSmart hides backup routes by default under the OMP best-path algorithm.
🔹 In networks with limited TLOC reachability, this can lead to blackholes.
🔹 The OMP Send-Backup-Paths option ensures that remote vEdges receive alternative routes.
🔹 Using the Send-Backup-Paths command, vSmart behaves more like a traditional IGP rather than a strict route-reflector.
Final Thoughts
Enabling OMP Send-Backup-Paths is a simple yet powerful fix to prevent blackholes in Cisco SD-WAN. If you're deploying SD-WAN in a multi-transport environment, this feature is critical to maintaining full reachability during failures.
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