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Layer 3 Networking
This section describes the concepts and choices to take into account when deciding on the configuration of Layer 3 networking.
Layer-3 architecture advantages
In layer-3 networking, routing takes instance MAC and IP addresses out of the network core, reducing state churn. The only time there would be a routing state change is in the case of a Top of Rack (ToR) switch failure or a link failure in the backbone itself. Other advantages of using a layer-3 architecture include:
- Layer-3 networks provide the same level of resiliency and scalability as the Internet.
- Controlling traffic with routing metrics is straightforward.
- You can configure layer-3 to useˇBGPˇconfederation for scalability. This way core routers have state proportional to the number of racks, not to the number of servers or instances.
- There are a variety of well tested tools, such as ICMP, to monitor and manage traffic.
- Layer-3 architectures enable the use of
quality of service (QoS)to manage network performance.
Layer-3 architecture limitations
The main limitation of layer-3 networking is that there is no built-in isolation mechanism comparable to the VLANs in layer-2 networks. Furthermore, the hierarchical nature of IP addresses means that an instance is on the same subnet as its physical host, making migration out of the subnet difficult. For these reasons, network virtualization needs to use IPencapsulation and software at the end hosts. This is for isolation and the separation of the addressing in the virtual layer from the addressing in the physical layer. Other potential disadvantages of layer 3 include the need to design an IP addressing scheme rather than relying on the switches to keep track of the MAC addresses automatically, and to configure the interior gateway routing protocol in the switches.