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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.

Letā€™s take a real-world example:



šŸ“ 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 doesnā€™t have an overlay tunnel to the Biz-Internet TLOC of vEdge-1.

šŸ”» 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.

šŸ”§ 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  


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

show omp routes 10.1.1.0/24  

Now, we see both primary and backup routes advertised.
2ļøāƒ£ Run a Traceroute:


traceroute 10.1.1.1  

Traffic now flows via vEdge-2 ā†’ Data Center.
Key Takeaways
šŸ”¹ vSmart hides backup routes by default under the OMP best-path algorithm.
šŸ”¹ In networks with limited TLOC reachability, this can lead to blackholes.
šŸ”¹ The OMP Send-Backup-Paths option ensures that remote vEdges receive alternative routes.
šŸ”¹ 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.
šŸ”” Subscribe for more SD-WAN insights!
#CiscoSDWAN #Networking #CCNP #CCIE #Routing #OMP #NetworkTroubleshooting #IT #vSmart #TLOC #BGP #OSPF

https://youtu.be/m4mLsvQNvmw

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