Cilium BGP Control Plane

Usage

Currently a single flag in the Cilium Agent exists to turn on the BGP Control Plane feature set.

--enable-bgp-control-plane=true

When set to true the BGP Control Plane Controllers will be instantiated and will begin listening for CiliumBGPPeeringPolicy events.

Currently, the BGP Control Plane will only work when IPAM mode is set to “cluster-pool”, “cluster-pool-v2”, and “kubernetes”

CiliumBGPPeeringPolicy CRD

All BGP peering topology information is carried in a CiliumBGPPeeringPolicy CRD.

CiliumBGPPeeringPolicy can be applied to one or more nodes based on its nodeSelector fields.

A Cilium node may only have a single CiliumBGPPeeringPolicy apply to it and if more than one does, it will apply no policy at all.

Each CiliumBGPPeeringPolicy defines one or more CiliumBGPVirtualRouter configurations.

When these CRDs are written or read from the cluster the Controllers will take notice and perform the necessary actions to drive the BGP Control Plane to the desired state described by the policy.

The policy in yaml form is defined below:

apiVersion: "cilium.io/v2alpha1"
kind: CiliumBGPPeeringPolicy
metadata:
 name: 01-bgp-peering-policy
spec: # CiliumBGPPeeringPolicySpec
 nodeSelector:
   matchLabels:
     bgp-policy: a
 virtualRouters: # []CiliumBGPVirtualRouter
 - localASN: 64512
   exportPodCIDR: true
   neighbors: # []CiliumBGPNeighbor
    - peerAddress: 'fc00:f853:ccd:e793::50/128'
      peerASN: 64512

Fields

nodeSelector: Nodes which are selected by this label selector will apply the given policy
 virtualRouters: One or more peering configurations outlined below. Each peering configuration can be thought of as a BGP router instance.
    virtualRouters[*].localASN: The local ASN for this peering configuration
    virtualRouters[*].exportPodCIDR: Whether to export the private pod CIDR block to the listed neighbors
    virtualRouters[*].neighbors: A list of neighbors to peer with
        neighbors[*].peerAddress: The address of the peer neighbor
        neighbors[*].peerASN: The ASN of the peer

Note

Setting unique configuration details of a particular instantiated virtual router on a particular Cilium node is explained in Virtual Router Attributes

Creating a BGP Topology

Rules

Follow the rules below to have a CiliumBGPPeeringPolicy correctly apply to a node.

• Only a single CiliumBGPPeeringPolicy can apply to a Cilium node.

  • If the BGP Control Plane on a node iterates through the CiliumBGPPeeringPolicy CRs currently written to the cluster and discovers (n > 1) policies match its labels, it will return an error and remove any existing BGP sessions. Only (n == 1) policies must match a node’s label sets.

  • Administrators should test a new BGP topology in a staging environment before making permanent changes in production.

• Within a CiliumBGPPeeringPolicy each CiliumBGPVirtualRouter defined must have a unique localASN field.

  • A node cannot host two or more logical routers with the same local ASN. Local ASNs are used as unique keys for a logical router.

  • A node can define the remote ASN on a per-neighbor basis to mitigate this scenario. See CiliumBGPNeighbor CR sub-structure.

• IPv6 single stack deployments must set an IPv4 encoded routerID field in each defined CiliumBGPVirtualRouter object within a CiliumBGPPeeringPolicy

  • Cilium running on a IPv6 single stack cluster cannot reliably generate a unique 32 bit BGP router ID, as it defines no unique IPv4 addresses for the node. The administrator must define these IDs manually or an error applying the policy will occur.

  • This is explained further in Virtual Router Attributes

Defining Topology

Within a CiliumBGPPeeringPolicy multiple CiliumBGPVirtualRouter(s) can be defined.

Each one can be thought of as a logical BGP router instance.

Defining more than one CiliumBGPVirtualRouter in a CiliumBGPVirtualRouter creates more than one logical BGP router on the hosts which the policy matches.

It is possible to create a single CiliumBGPPeeringPolicy for all nodes by giving each node in a cluster the same label and defining a single CiliumBGPPeeringPolicy which applies to this label.

It is also possible to provide each Kubernetes node its own CiliumBGPPeeringPolicy by giving each node a unique label and creating a CiliumBGPPeeringPolicy for each unique label.

This allows for selecting subsets of nodes which peer to a particular BGP router while another subset of nodes peer to a separate BGP router, akin to an “AS-per-rack” topology.

Virtual Router Attributes

A CiliumBGPPeeringPolicy can apply to multiple nodes.

When a CiliumBGPPeeringPolicy applies to one or more nodes each node will instantiate one or more BGP routers as defined by the list of CiliumBGPVirutalRouter.

However, there are times where fine-grained control over an instantiated virtual router’s configuration needs to take place.

To accomplish this a Kubernetes annotation is defined which applies to Kubernetes Node resources.

A single annotation is used to specify a set of configuration attributes to apply to a particular virtual router instantiated on a particular host.

The syntax of the annotation is as follows:

cilium.io/bgp-virtual-router.{asn}="key=value,..."

The {asn} portion should be replaced by the virtual router’s local ASN you wish to apply these configuration attributes to.

The following sections outline the currently supported attributes.

Note

Each following section describes the syntax of applying a single attribute, however the annotation’s value supports a comma separated lists of attributes and applying multiple attributes in a single annotation is supported.

Note

When duplicate key=value attributes are defined the last one will be selected.

Router ID Attribute

When Cilium is running on an IPv4 or a dual-stack IPv4/6 cluster the BGP Control Plane will utilize the IPv4 addressed used by Cilium for external reach ability.

This will typically be Kubernetes’ reported external IP address but can also be configured with a Cilium agent flag.

When running in IPv6 single stack or when the administrator needs to manually define the instantiated BGP server’s router ID a Kubernetes annotation can be placed on the node.

The annotation takes the following syntax:

cilium.io/bgp-virtual-router.{asn}="router-id=127.0.0.1"

The above annotation syntax should replace {asn} with the local ASN of the CiliumBGPVirtualRouter you are setting the provided router ID for.

When the BGPControlPlane evaluates a CiliumBGPPeeringPolicy with a CiliumBGPVirtualRouter it also searches for an annotation which targets the aforementioned CiliumBGPVirtualRouter local ASN.

If found it will use the provided router ID and not attempt to use the IPv4 address assigned to the node.

Local Listening Port

By default the GoBGP BGPRouterManager will instantiate each virtual router without a listening port.

It is possible to deploy a virtual router which creates a local listening port where BGP connections may take place.

If this is desired the following annotation can be provided

cilium.io/bgp-virtual-router.{asn}="local-port=45450"

Architecture

The BGP Control Plane is split into a Agent-Side Control Plane and a Operator-Side control plane (not yet implemented).

Both control planes are implemented by a Controller which follows the Kubernetes controller pattern.

Both control planes primary listen for CiliumBGPPeeringPolicy CRDs, long with other Cilium and Kubernetes resources useful for implementing a BGP control plane.

Agent-Side Architecture

Controller

The Agent-Side Control Plane implements a controller located in pkg/bgpv1/agent/controller.go.

The controller listens for CiliumBGPPeeringPolicy, determines if a policy applies to its current host and if it does, captures some information about Cilium’s current state then calls down to the implemented BGPRouterManager.

BGPRouterManager

The BGPRouterManager is an interface used to define a declarative API between the Controller and instantiated BGP routers.

The interface defines a single declarative method whose argument is the desired CiliumBGPPeeringPolicy (among a few others).

The BGPRouterManager is in charge of pushing the BGP Control Plane to the desired CiliumBGPPeeringPolicy or returning an error if it is not possible.

GoBGP Implementation

The first implementation of BGPRouterManager utilizes the gobgp package.

You can find this implementation in pkg/bgpv1/gobgp.

This implementation will - evaluate the desired CiliumBGPPeeringPolicy - create/remove the desired BGP routers - advertise/withdraw the desired BGP routes - enable/disable any BGP server specific features - inform the caller if the policy cannot be applied

The GoBGP implementation is capable of evaluating each CiliumBGPVirtualRouter in isolation.

This means when applying a CiliumBGPPeeringPolicy the GoBGP BGPRouterManager will attempt to create each CiliumBGPVirtualRouter.

If a particular CiliumBGPVirtualRouter fails to instantiate the error is logged and the BGPRouterManager will continue to the next CiliumBGPVirtualRouter, utilizing the aforementioned logic.

GoBGP BGPRouterManager Architecture

It’s worth expanding on how the gobgp implementation of the BGPRouterManager works internally.

This BGPRouterManager views each CiliumBGPVirtualRouter as a BGP router instance.

Each CiliumBGPVirtualRouter defines a local ASN, a router ID and a list of CiliumBGPNeighbors to peer with.

This is enough for the BGPRouterManager to create a BgpServer instance, which is the nomenclature defining a BGP speaker in gobgp-package-parlance.

This BGPRouterManager groups BgpServer instances by their local ASNs.

This leads to the following rule: - A CiliumBGPPeeringPolicy applying to node A must not have two or more CiliumBGPVirtualRouters with the same localASN fields.

The gobgp BGPRouterManager employs a set of ConfigReconcilerFunc(s) which perform the order-dependent reconciliation actions for each BgpServer it must reconcile.

A ConfigReconcilerFunc is simply a function with a typed signature.

type ConfigReconcilerFunc func(ctx context.Context, m *BGPRouterManager, sc *ServerWithConfig, newc *v2alpha1api.CiliumBGPVirtualRouter, cstate *agent.ControlPlaneState) error

See the source code at pkg/bgpv1/gobgp/reconcile.go for a more in depth explanation of how each ConfigReconcilerFunc is called.