Simple setups with Calico don’t require anything special, you can just install and configure MetalLB as usual and enjoy.
However, if you are using Calico’s external BGP peering capability to advertise your cluster prefixes over BGP, and also want to use BGP in MetalLB, you will need to jump through some hoops.
BGP only allows one session to be established per pair of nodes. So, if Calico has a session established with your BGP router, MetalLB cannot establish its own session – it’ll get rejected as a duplicate by BGP’s conflict resolution algorithm.
Unfortunately, Calico does not currently provide the extension points we would need to make MetalLB coexist peacefully. There are bugs filed with Calico to add these extension points, but in the meantime, we can only offer some hacky workarounds.
If you are deploying to a cluster using a traditional “rack and spine” router architecture, you can work around the limitation imposed by BGP with some clever choice of peering.
Let’s start with the network architecture, and see how we can add in MetalLB:
In this architecture, we have 4 machines in our Kubernetes cluster, spread across 2 racks. Each rack has a top-of-rack (ToR) router, and both ToRs connect to an upstream “spine” router.
The arrows represent BGP peering sessions: Calico has been configured to not automatically mesh with itself, but to instead peer with the ToRs. The ToRs in turn peer with the spine, which propagates routes throughout the cluster.
Ideally, we would like MetalLB to connect to the ToRs in the same way that Calico does. However, Calico is already “consuming” the one allowed BGP session between machine and ToR.
The alternative is to make MetalLB peer with the spine router(s):
Properly configured, the spine can redistribute MetalLB’s routes to anyone that needs them. And, because there are no preexisting BGP sessions between the machines and the spine, there is no conflict between Calico and MetalLB.
The downside of this option is additional configuration complexity, and a loss of scalability: instead of scaling the number of spine BGP sessions by the number of racks in your cluster, you’re once again scaling by the total number of machines. In some deployments, this may not be acceptable.
In large clusters, another compromise might be to dedicate only
certain racks to externally facing services: constrain the MetalLB
speaker daemonset to schedule only on those racks, and either use the
externalTrafficPolicy, or also constrain the pods of the
externally facing services to run on those racks.
If your networking hardware supports VRFs (Virtual Routing and Forwarding), you may be able to “split” your router in two, and peer Calico and MetalLB to separate halves of the same router. Then, with judicious inter-VRF route leaking, you can re-merge the two routing tables.
While this should theoretically work, it hasn’t been demonstrated, and setting it up varies wildly based on which routing software/hardware you are interfacing with. If you get this working, please let us know, especially if you have tips on how to make this work best!
None of these workarounds is very satisfying. Until Calico supports more advanced configuration for BGP peers, we are stuck doing acrobatics to integrate the two.
If you have an idea for another workaround that would enable Calico and MetalLB to coexist nicely, please tell us !