Notably, vendor-specific APIs have arisen in the southbound direction in addition to OF. This is due in part to OF's limited set of commands and sometimes-difficult implementation in legacy silicon. Despite supporting OpenFlow, Cisco is an example of a vendor emphasizing APIs via its ONE initiative, arguing that its APIs allow network programmers to take full advantage of the capabilities of their hardware.
Another term that comes up frequently in SDN conversations is that of overlay networks. Simply stated, overlays are used to create virtual network containers that are logically isolated from one another while sharing the same underlying physical network.
Network engineers familiar with commonly deployed Generic Routing Encapsulation (GRE) will grasp the overlay concept readily. One packet (or frame) is encapsulated inside of another one; the encapsulated packet is forwarded to a tunnel endpoint where it is decapsulated. The original packet is then delivered to its destination. Overlays leverage this "packet in a packet" technique to securely hide networks from one another and traverse network segments that would otherwise be barriers. Layer 2 extension and multi-tenancy are popular use-case for overlays.
A number of overlay protocols have been released and promoted by standards bodies during the last few years, driven by a virtualized data center's ability to move a host anywhere at anytime. Some SDN controllers use overlays as their transport of choice to build a bridge between hosts scattered across the data center; soft switches usually serve as either end of the tunnel. Virtual eXtensible LAN (VXLAN) has the broadest industry support at this time, with Cisco, Brocade and VMware among others committed to the overlay. Termination of VXLAN tunnels in hardware is supported by switches from Arista and Brocade. Hardware termination of VXLAN underscores the groundswell of industry adoption, as overlays are usually terminated by software switches.
VXLAN encapsulates Layer 2 frames inside of a Layer 3 UDP packet. This allows hosts inside of a VXLAN segment to communicate with each other as if they were on the same Layer 2 network, even though they might be separated by one or more Layer 3 networks.
In addition, since VXLAN preserves the entire Layer 2 frame, VLAN tags are preserved, allowing for multiple Layer 3 networks to exist inside of a VXLAN segment. Customers (also known as tenants) inside the VXLAN segment see a network much like any they are used to, while the underlying network only sees VXLAN packets identified by a segment ID.
Each VXLAN network is identified by a segment ID in the VXLAN header; this ID is 24 bits long, allowing for 16 million tenants to share the same network infrastructure while staying isolated from one another.
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