BGP Control Plane Operation Guide

This document provides guidance on how to operate the BGP Control Plane.

Logs

BGP Control Plane logs can be found in the Cilium agent logs. The logs are tagged with subsys=bgp-control-plane. You can use this tag to filter the logs as in the following example:

kubectl -n kube-system logs <cilium agent pod name> | grep "subsys=bgp-control-plane"

Metrics

Metrics exposed by BGP Control Plane are listed in the metrics document.

Restarting an Agent

When you restart the Cilium agent, the BGP session will be lost because the BGP speaker is integrated within the Cilium agent. The BGP session will be restored once the Cilium agent is restarted. However, while the Cilium agent is down, the advertised routes will be removed from the BGP peer. As a result, you may temporarily lose connectivity to the Pods or Services. You can enable the Graceful Restart to continue forwarding traffic to the Pods or Services during the agent restart.

Upgrading or Downgrading Cilium

When you upgrade or downgrade Cilium, you must restart the Cilium agent. For more details about the agent restart, see Restarting an Agent section.

Note that with BGP Control Plane, it’s especially important to pre-pull the agent image by following the preflight process before upgrading Cilium. Image pull is time-consuming and error-prone because it involves network communication. If the image pull takes longer, it may exceed the Graceful Restart time (restartTimeSeconds) and cause the BGP peer to withdraw routes.

Shutting Down a Node

When you need to shut down a node for maintenance, you can follow the steps below to avoid packet loss as much as possible.

Drain the node to evict all workloads. This will remove all Pods on the node from the Service endpoints and prevent Services with externalTrafficPolicy=Cluster from redirecting traffic to the node.
```
kubectl drain <node-name> --ignore-daemonsets
```
Deconfigure the BGP sessions by modifying or removing the CiliumBGPPeeringPolicy node selector label on the Node object. This will shut down all BGP sessions on the node.
```
# Assuming you select the node by the label enable-bgp=true
kubectl label node <node-name> --overwrite enable-bgp=false
```
Wait for a while until the BGP peer removes routes towards the node. During this period, the BGP peer may still send traffic to the node. If you shut down the node without waiting for the BGP peer to remove routes, it will break the ongoing traffic of externalTrafficPolicy=Cluster Services.
Shut down the node.

In step 3, you may not be able to check the peer status and may want to wait for a specific period of time without checking the actual peer status. In this case, you can roughly estimate the time like the following:

If you disable the BGP Graceful Restart feature, the BGP peer should withdraw routes immediately after step 2.
If you enable the BGP Graceful Restart feature, there are two possible cases.
- If the BGP peer supports the Graceful Restart with Notification (RFC 8538), it will withdraw routes after the Stale Timer (defined in the RFC 8538#section-4.1) expires.
- If the BGP peer does not support the Graceful Restart with Notification, it will withdraw routes immediately after step 2 because the BGP Control Plane sends the BGP Notification to the peer when you unselect the node.

The above estimation is a theoretical value, and the actual time always depends on the BGP peer’s implementation. Ideally, you should check the peer router’s actual behavior in advance with your network administrator.

Warning

Even if you follow the above steps, some ongoing Service traffic originally destined for the node may be reset because, after the route withdrawal and ECMP rehashing, the traffic is redirected to a different node, and the new node may select a different endpoint.

Failure Scenarios

This document describes common failure scenarios that you may encounter when using the BGP Control Plane and provides guidance on how to mitigate them.

Cilium Agent Down

If the Cilium agent goes down, the BGP session will be lost because the BGP speaker is integrated within the Cilium agent. The BGP session will be restored once the Cilium agent is restarted. However, while the Cilium agent is down, the advertised routes will be removed from the BGP peer. As a result, you may temporarily lose connectivity to the Pods or Services.

Mitigation

The recommended way to address this issue is by enabling the Graceful Restart feature. This feature allows the BGP peer to retain routes for a specific period of time after the BGP session is lost. Since the datapath remains active even when the agent is down, this will prevent the loss of connectivity to the Pods or Services.

When you can’t use BGP Graceful Restart, you can take the following actions, depending on the kind of routes you are using:

PodCIDR routes

If you are advertising PodCIDR routes, pods on the failed node will be unreachable from the external network. If the failure only occurs on a subset of the nodes in the cluster, you can drain the unhealthy nodes to migrate the pods to other nodes.

Service routes

If you are advertising service routes, the load balancer (KubeProxy or Cilium KubeProxyReplacement) may become unreachable from the external network. Additionally, ongoing connections may be redirected to different nodes due to ECMP rehashing on the upstream routers. When the load balancer encounters unknown traffic, it will select a new endpoint. Depending on the load balancer’s backend selection algorithm, the traffic may be directed to a different endpoint than before, potentially causing the connection to be reset.

If your upstream routers support ECMP with Resilient Hashing, enabling it may help to keep the ongoing connections forwarded to the same node. Enabling the Maglev Consistent Hashing feature in Cilium may also help since it increases the probability that all nodes select the same endpoint for the same flow. However, it only works for the externalTrafficPolicy: Cluster. If the Service’s externalTrafficPolicy is set to Local, it is inevitable that all ongoing connections with the endpoints on the failed node, and connections forwarded to a different node than before, will be reset.

Node Down

If the node goes down, the BGP sessions from this node will be lost. The peer will withdraw the routes advertised by the node immediately or takes some time to stop forwarding traffic to the node depending on the Graceful Restart settings. The latter case is problematic when you advertise the route to a Service with externalTrafficPolicy=Cluster because the peer will continue to forward traffic to the unavailable node until the restart timer (which is 120s by default) expires.

Mitigation

Involuntary Shutdown

When a node is involuntarily shut down, there’s no direct mitigation. You can choose to not use the BGP Graceful Restart feature, depending on the trade-off between the failure detection time vs stability provided by graceful restart in cases of Cilium pod restarts.

Disabling the Graceful Restart allows the BGP peer to withdraw routes faster. Even if the node is shut down without BGP Notification or TCP connection close, the worst case time for peer to withdraw routes is the BGP hold time. When the Graceful Restart is enabled, the BGP peer may need hold time + restart time to withdraw routes received from the node.

Voluntary Shutdown

When you voluntarily shut down a node, you can follow the steps described in the Shutting Down a Node section to avoid packet loss as much as possible.

Peering Link Down

If the peering link between the BGP peers goes down, usually, both the BGP session and datapath connectivity will be lost. However, there may be a period during which the datapath connectivity is lost while the BGP session remains up and routes are still being advertised. This can cause the BGP peer to send traffic over the failed link, resulting in dropped packets. The length of this period depends on which link is down and the BGP configuration.

If the link directly connected to the Node goes down, the BGP session will likely be lost immediately because the Linux kernel detects the link failure and shuts down the TCP session right away. If a link not directly connected to the Node goes down, the BGP session will be lost after the hold timer expires, which is set to 90 seconds by default.

Mitigation

To make link detection failure fast, you can adjust holdTimeSeconds and keepAliveTimeSeconds in the BGP configuration to the shorter value. However, the minimal possible values are holdTimeSeconds=3 and keepAliveTimeSeconds=1. The general approach to make failure detection faster is to use BFD (Bidirectional Forwarding Detection), but currently, Cilium does not support it.

Cilium Operator Down

If the Cilium Operator goes down, PodCIDR allocation by IPAM, and LoadBalancer IP allocation by LB-IPAM are stopped. Therefore, the advertisement of new and withdrawal of old PodCIDR and Service VIP routes will be stopped as well.

Mitigation

There’s no direct mitigation in terms of the BGP. However, running the Cilium Operator with a high-availability setup will make the Cilium Operator more resilient to failures.

Service Losing All Backends

If all service backends are gone due to an outage or a configuration mistake, BGP Control Plane behaves differently depending on the Service’s externalTrafficPolicy. When the externalTrafficPolicy is set to Cluster, the Service’s VIP remains advertised from all nodes selected by the CiliumBGPPeeringPolicy. When the externalTrafficPolicy is set to Local, the advertisement stops entirely because the Service’s VIP is only advertised from the node where the Service backends are running.

Mitigation

There’s no direct mitigation in terms of the BGP. In general, you should prevent the Service backends from being all gone by Kubernetes features like PodDisruptionBudget.