Virtual Extensible Local Area Network (VXLAN)

This book focuses primarily on VXLAN, an overlay technology that helps address scalability issues with VLANs. VXLAN encapsulates layer 2 Ethernet frames inside layer 4 UDP packets that can be forwarded or routed between hosts. This means that a virtual network can be transparently extended across a large network without any changes to the end hosts. In the case of OpenStack Networking, however, a VXLAN mesh network is commonly constructed only between nodes that exist in the same cloud.

Rather than use VLAN IDs to differentiate between networks, VXLAN uses a VXLAN Network Identifier (VNI) to serve as the unique identifier on a link that potentially carries traffic for tens of thousands of networks, or more. An 802.1q VLAN header supports up to 4,096 unique IDs, whereas a VXLAN header supports approximately 16 million unique IDs. Within an OpenStack cloud, virtual machine instances are unaware that VXLAN is used to forward traffic between hosts. The VXLAN Tunnel Endpoint (VTEP) on the physical node handles the encapsulation and decapsulation of traffic without the instance ever knowing.

Because VXLAN network traffic is encapsulated, many network devices cannot participate in these networks without additional configuration, if at all. As a result, VXLAN networks are effectively isolated from other networks in the cloud and require the use of a Neutron router to provide access to connected instances. More information on creating Neutron routers begins in Chapter 10, Creating Standalone Routers with Neutron.

While not as performant as VLAN or flat networks on some hardware, the use of VXLAN is becoming more popular in cloud network architectures where scalability and self-service are major drivers. Newer networking hardware that offers VXLAN offloading capabilities should be leveraged if you are considering implementing VXLAN-based overlay networks in your cloud.

More information on how VXLAN encapsulation works is described in RFC 7348, available at the following URL: https://tools.ietf.org/html/rfc7348