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Solving complex Network Function  integrations with Host Based Routing 

By July 8, 2025No Comments

Problem to tackle 

In modern cloud-native environments, like with Sylva, Kubernetes cluster management  benefits from a high degree of automation. However, achieving similar flexibility and  automation at the datacenter network layer—particularly within IP and VLAN switch fabrics— remains a significant challenge. Network Functions typically have very complex network  connectivity profile due to the need to integrate with many existing partner systems, often  part of telco legacy domain. In such context, integration of Network Functions turns to be  laborious and error prone task on the datacenter network fabric that nobody is happy with:  not the Network Function team, not the platform team, not the network team.  

Host Based Routing solution 

Sylva recognized this problem early on and already in 2024 started experimenting with Host  Based Routing (HBR), a concept which foresees containerized router instance, on every  Kubernetes host (node). The node then directly peers with EVPN/VXLAN capable fabric,  thanks to BGP control plane capabilities of the containerized router. This is complete change  comparing to the traditional approach to connecting the nodes to the network fabric with  static VLAN configuration. Nodes become integral part of fabric and participate directly in  routing decisions. This opens possibility to use Kubernetes Resource Model (KRM) to  configure all necessary connections to different VPNs / VRFs and target networks in the data  center network, including route leaking configurations, solely within the Kubernetes cluster,  without touching the network. In this way the network can remain relatively static from the  perspective of day 2 operations as entire relevant network configuration remains within Sylva  cluster. This containerized router uses dynamic properties of BGP protocol flexible and  declarative connectivity management to answer of Telco Workloads connectivity needs.

In Host Based Routing connectivity demand is declaratively configured through Kubernetes  Custom resources (1). They are reconciled by Host Network Operator which uses standard  interfaces (2) to translate this demand to the configuration of the Containerized Routing  Agent (CRA), which creates necessary logical interfaces on the node and programs routing  table in the Netlink level of Linux Kernel (3), based on which necessary connectivity is  established on layer 3 towards the network fabric (4) and layer 2 towards the pods/containers  (5). This eliminates need for complicated network automation since the main use cases for it  are covered within Kubernetes cluster. Furthermore, it enables flexible and dynamic  allocation of the compute nodes within data center, without constraints imposed by pre configured switches. 

HBR Origins and Evolution 

Originally conceptualized and developed internally by Deutsche Telekom and based on Free  Range Router (FRR) as CRA, HBR has been in production since 2022. It is default  networking and connectivity solution for more than 300 bare metal Kubernetes clusters which  host critical network functions like 5G SA Core, enabling zero touch configuration of the most  complex connectivity scenarios.  

To extend this ecosystem a collaboration within Sylva has been initiated by Deutsche  Telekom, 6WIND and Orange to prove usability and viability of HBR beyond its use case in  Deutsche Telekom. The Proof-of-Concept has been organized hands on in Sylva Validation  Center in Paris. It was executed in two phases which were recently successfully closed.

The First success in this collaboration was integration of 6WIND’s Virtual Service Router  (VSR) as Containerized Routing Agent within the HBR setup, however without full integration  with Host Network Operator. The goal was to show that enterprise grade virtual router like  VSR, can easily take the role of CRA and even enable advanced functionalities. 

“As virtual router pioneer and early contributor to Free Range Routing project, we have been  keen to show that our product VSR can successfully work in HBR solution and add new  possibilities on top of providing connectivity, such as filtering and egress capabilities” – Jean  Mickaël Guerin, 6WIND CTO 

“We have implemented EVPN/VXLAN based fabric within our lab, deployed HBR solution  and tested numerous scenarios and complex connectivity setups both functionally and  performance wise over several months. Not only they all worked very well, but during that  entire time we did not need to touch the network fabric configuration even once.” – Vlad  Onutu, Orange Cloud Network Lead 

In second phase the latest HBR version 2.0 was deployed including full integration of host  network operator. This time using FRR-CRA reference implementation. The goal was to  demonstrate declarative model of fulfilling connectivity demand via HBR custom resources and possibilities which comes with it (e.g. GitOps based configuration or connectivity self service). 

“We have recently released version 2.0 of HBR which enables seamless pluggability of  different CRAs (e.g. enterprise versions), by offering stable integration API with host network  operator. It also contains open-source reference implementation based on FRR container, to  facilitate experimentation and testing. The phase two of HBR PoC was proof for us that HBR  is mature enough to be used in cloud environments beyond Deutsche Telekom, like Sylva.” – Christopher Dziomba, Lead HBR Architect, Deutsche Telekom.  

“HBR 2.0 solution proved to be easy to install and use, things working after the setup of the  values. Pods with macvlan interfaces in various vlans can communicate by leveraging the  EVPN/VXLAN fabric built between the cluster (frr-cra pods) and the switches.” – Vlad Onutu,  Orange Cloud Network Lead 

Next steps 

  • Deutsche Telekom as the founding member of Sylva intends to move the HBR  development activities to Sylva and put the code on disposal under Apache 2.0  license, fostering open collaboration and adoption. 
  • Extension of HBR capabilities from IP layer (BGP) routing to Layer 2 VLAN  automation is planned in order to enable usage of part of its functionalities in  traditional network fabric designs. 

Takeaway 

HBR aims to enhance Telco Cloud environments by automating network connectivity for  Telco workloads and enabling seamless IP datacenter interconnections (BGP-based).