bgpv1

Cilium BGP Control Plane

BGP Control Plane provides a way for Cilium to advertise routes to connected routers by using the Border Gateway Protocol (BGP). BGP Control Plane makes Pod networks and/or Services of type LoadBalancer reachable from outside the cluster for environments that support BGP. Because BGP Control Plane does not program the datapath, do not use it to establish reachability within the cluster.

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.

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
      eBGPMultihopTTL: 10
      connectRetryTimeSeconds: 120
      holdTimeSeconds: 90
      keepAliveTimeSeconds: 30
      gracefulRestart:
        enabled: true
        restartTimeSeconds: 120

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[*].peerPort: Optional TCP port number of the neighbor. 1-65535 are valid values and defaults to 179 when unspecified.
		neighbors[*].peerASN: The ASN of the peer	
		neighbors[*].eBGPMultihopTTL: Time To Live (TTL) value used in BGP packets. The value 1 implies that eBGP multi-hop feature is disabled.
		neighbors[*].connectRetryTimeSeconds: Initial value for the BGP ConnectRetryTimer (RFC 4271, Section 8). Defaults to 120 seconds.
		neighbors[*].holdTimeSeconds: Initial value for the BGP HoldTimer (RFC 4271, Section 4.2). Defaults to 90 seconds.
		neighbors[*].keepAliveTimeSeconds: Initial value for the BGP KeepaliveTimer (RFC 4271, Section 8). Defaults to 30 seconds.
		neighbors[*].gracefulRestart.enabled: The flag to enable graceful restart capability.
		neighbors[*].gracefulRestart.restartTimeSeconds: The restart time advertised to the peer (RFC 4724 section 4.2).

Note: Setting unique configuration details of a particular instantiated virtual router on a particular Cilium node is explained in [Virtual Router Attributes](#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.

flowchart LR
	subgraph K ["Kubernetes Cluster"]
		subgraph P1 ["CiliumBGPPeeringPolicy [Label=A]"]
			node1("Cilium-01 [Label=A,B]"):::error
		end
		subgraph P2 ["CiliumBGPPeeringPolicy [Label=B]"]
			node2("Cilium-01 [Label=A,B]"):::error
		end
	end
classDef error fill:darkred,color:white;
classDef kBg fill:steelblue,color:white;
classDef pBg fill:lightblue;
class K kBg;
class P1,P2 pBg

The above diagram demonstrates the aforementioned error case. Cilium-01 will not apply any CiliumBGPPeeringPolicy since both policies match its label set.

• 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](#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.

This is illustrated in the diagram below with a policy that peers to a single BGP router.

	flowchart TB
		subgraph A["Kubernetes Cluster"]
			subgraph id2 ["CiliumBGPPeeringPolicy Selector:[Label=A]"]
				direction LR
				node1("Cilium-01 Label=A"):::n
				node2("Cilium-02 Label=A"):::n
				node3("Cilium-03 Label=A"):::n
			end
		end
		rt((BGP Router)):::r
		node1 -->|peer| rt
		node2 -->|peer| rt
		node3 -->|peer| rt
	classDef n fill:lightblue;
	classDef r fill:lightyellow;
	classDef p fill:steelblue,color:white;
	class id2 p

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 is illustrated in the diagram below with each node is provided its own policy which peers to their own

flowchart TB
	subgraph A["Kubernetes Cluster"]
		subgraph id3 ["CiliumBGPPeeringPolicy Selector:[Label=C]"]
			node3("Cilium-03 Label=C"):::n
		end
		direction TB
		subgraph id2 ["CiliumBGPPeeringPolicy Selector:[Label=B]"]
			node2("Cilium-02 Label=B"):::n
		end
		subgraph id1 ["CiliumBGPPeeringPolicy Selector:[Label=A]"]
			node1("Cilium-01 Label=A"):::n
		end
	end
	rt1((BGP Router-01)):::r
	rt2((BGP Router-02)):::r
	rt3((BGP Router-03)):::r
	node1 -->|peer| rt1
	node2 -->|peer| rt2
	node3 -->|peer| rt3
classDef n fill:lightblue;
classDef r fill:lightyellow;
classDef p fill:steelblue,color:white,font-size:11px;
class id1,id2,id3 p

By showing a unified topology (former diagram) and a segregated topology (latter diagram) it should be apparent that any combination of the two is possible.

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

Component Diagram

sequenceDiagram
	participant K as Kubernetes
	participant C as Controller
	participant I as SharedInformers
	participant M as BGPRouterManager

	K ->> C: CiliumBGPPeeringPolicy event
	C ->> I: Query shared informers for current cached state
	C ->> C: Evaluate if any reconciliation is necessary
	alt reconciliation is necessary
		C ->> M: Call down to BGPRouterMananger
		M ->> M: Reconcile configured BGP servers
	else reconcilation is not necessary
		C ->> C: Sleep until next event
	end

Above is a high level sequence diagram describing the control flow of the Agent-Side BGP Control Plane

flowchart 
    subgraph agent
        agent:node_spec("node event")
        agent:policy_spec("policy event")
        agent:signal("control loop")
    end
    agent:node_spec---agent:signal
    agent:policy_spec---agent:signal

    subgraph manager
        manager:router_manager("router manager")
        manager:reconcilers("reconcilers")
        manager:servers("server configs")
    end
    agent:signal---manager:router_manager
    manager:router_manager-->manager:servers
    manager:router_manager-->manager:reconcilers

    subgraph types
        types:bgp("cilium bgp")
        types:router("bgp interface")
    end

    subgraph gobgp
    gobgp:gobgp("gobgp client")
    end

    manager:reconcilers-->gobgp:gobgp

Note: We summarize the Kubernetes events which trigger the Controller to just a CiliumBGPPeeringPolicy event, however the Controller will wake on other events which influence changes in the Agent-Side BGP Control Plane. See the source code for full details.

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 Manager.

Manager

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

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

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 Manager implementation is capable of evaluating each CiliumBGPVirtualRouter in isolation.

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

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

It's worth expanding on how the implementation of the Manager works internally.

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

This Manager 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 Manager employs a set of ConfigReconciler(s) which perform the order-dependent reconciliation actions for each BgpServer it must reconcile.

A ConfigReconciler is an interface where the reconciler is invoked via the Reconcile method.

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

Router

Underlying router implementation exposes imperative API for BGP related configuration, such as add/remove neighbor, add/remove routes etc. Currently, only gobgp is supported as underlying routing implementation.

This shim layer provides translation between cilium specific BGP types and gobgp types.

See the source code at pkg/bgpv1/gobgp for more details.