Use Cillium for the CNI (#12)

Co-authored-by: Steffen Petersen <steffen.petersen@eficode.com>

docs/hardware_ready/ADRs/Cilium_as_network_plugin.md

@@ -0,0 +1,45 @@
+---
+title: Use Cilium as Network Plugin
+---
+
+| status: | date: | decision-makers: |
+| --- | --- | --- |
+| proposed | 2025-02-18 | Alexandra Aldershaab, Steffen Petersen |
+
+
+## Context and Problem Statement
+
+A CNI plugin is required to implement the Kubernetes network model by assigning IP addresses from preallocated CIDR ranges 
+to pods and nodes. The CNI plugin is also responsible for enforcing network policies that control how traffic flows between 
+namespaces as well as between the cluster and the internet.
+
+## Considered Options
+
+* Flannel
+* Cilium
+* Calico
+
+## Decision Outcome
+
+Chosen option: **Cilium**, because it is a fully conformant CNI plugin that works in both cloud and on-premises environments
+while also providing support for network policies as well as more advanced networking features. Cilium has also gained 
+rapid adoption in the Kubernetes community and is considered the future standard of CNI plugins.
+
+Flannel was considered, but it does not support network policies which is considered a hard requirement.
+
+Calico, while supporting Network policies, falls short compared to Cilium in terms of advanced networking features.
+
+### Consequences
+
+* Good, because Cilium provides support for network policies on L7 as well as the usual L3/L4.
+* Good, because Cilium provides support for BGP controlplane integration, allowing for seamless integration with existing
+  networking infrastructure.
+* Good, because Cilium provides a feature called Egress Gateway which allows for traffic exiting the cluster to be routed 
+  through specific nodes, facilitating smooth integration with existing security infrastructure such as IP-based firewalls.
+* Good, because Cilium comes with a utility called Hubble which provides deep observability into the network traffic, allowing
+  for easy debugging and troubleshooting of network issues.
+
+* Bad, because Cilium requires you to understand both Kubernetes networking and tradition networking concepts to fully utilize
+  its advanced features.
+* Bad, because Cilium does not come installed by default on any flavor of Kubernetes, requiring additional steps to 
+  install it and provide necessary custom configuration.

docs/hardware_ready/_index.md

@@ -16,4 +16,4 @@ In case virtualisation is chosen, the below recommendations are what you would r
 | Kubernetes Node Operating System | The Operating System running on each of the hosts that will be part of your Kubernetes cluster | Choosing the right OS will be the foundation for building a production-grade Kubernetes cluster |  |
 | Storage solution | The underlying storage capabilities which Kubernetes will leverage to provide persistence for stateful workloads | Choosing the right storage solution for your clusters needs is important as there is a lot of balance tradeoffs associated with it, e.g redundancy vs. complexity |  |
 | Container Runtime (CRI) | The software that is responsible for running containers | You need a working container runtime on each node in your cluster, so that the kubelet can launch pods and their containers |  |
-| Network plugin (CNI) | Plugin used for cluster networking | A CNI plugin is required to implement the Kubernetes network model |  |
+| Network plugin (CNI) | Plugin used for cluster networking | A CNI plugin is required to implement the Kubernetes network model | [Cilium](Cilium_as_network_plugin.md) |