Chapter 9. Layer 4 Load Balancing

The load balancer service in MidoNet provides Layer 4 (L4) load balancing. A typical use case involves a tenant wishing to balance traffic (load) from floating IP addresses to private IP addresses.

Generally, the traffic comes in from publicly routable addresses (for example, from users of the service all over the world) to a virtual IP address (VIP) (an IP assigned to the load balancer instead of a VM), and is then load-balanced to numerous private IP addresses within the cloud. The load balancer sends the traffic to the private IP addresses of the back-end servers – in MidoNet’s case, these back-end servers are VMs. The load balancer does not terminate the connection, nor is it really transparent because it’s translating the destination IP.

Configuration overview

As part of the configuration, you define a pool of back-end servers (VMs) to which traffic is load-balanced. The pool members typically have private IP addresses. The placement of the VMs is flexible but must take into account where the load balancer is configured. A general rule is that the VM private addresses must be reachable from the router to which the load balancer is attached. Therefore, pool members may

  • all belong to a single subnet of the router, or
  • they may be placed in two or more subnets of the router

Finally, note that because the load balancer leaves the request source address unmodified, the load balancer must be placed in the path of the return traffic – otherwise return traffic will go directly to the request source without giving the load balancer a chance to reverse the VIP→back-end-IP translation that was applied on the forward packets.

Health monitoring

In addition, you can configure a health monitor to perform checks on the back-end servers. The health monitor automatically removes unhealthy nodes from the pool and adds them back when they return to health.

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