What is “virtual” – not reality?
This idea could launch a philosophical discussion. It harkens back to Rene Descartes proving his very being with the proclamation: "I think, therefore I exist" (“Cogito, ergo sum”) and laying the groundwork for further proof of the surrounding material world and the necessary connectivity of "things" (or metaphysics).
Webster's defines “virtual” as "very close to being something without actually being it."
Virtualization sprang from computing as far back as the 60's describing methods of logical system resource division among different applications in mainframes. It has since grown to encompass a variety of computing concepts, such as software virtualization, database virtualization, storage virtualization, network virtualization, etc. In the early 90’s, virtual reality (considered by many to be the holy grail of computing) was popularized in the movie "Lawn Mower Man." However, the concept can be traced back to Charles Wheatstone's stereoscope research in 1838, which demonstrated that the brain processes individual two dimensional images as a single three dimensional image, albeit with the aid of a stereoscope. In the present day, think Imax, Google Cardboard, or Oculus Rift.
Network Virtualization is not a concept born yesterday. Consider that in 1981 Dr. David Sincoskie was busy experimenting with segmenting voice over Ethernet broadcast networks with tagging before Radia Perlman had even invented the Spanning Tree Protocol in 1985 to facilitate fault tolerance and redundant paths. It wasn't until 1990 that the IEEE adopted 802.1D, and it wasn’t until 1998 that 802.1Q was ratified. By the turn of the century, switched networks dominated the landscape, supplanting the reliance on hubs, repeaters, and bridges and making virtual LANs (VLANs) commonplace. A LAN without VLANs is virtually unthinkable nowadays.
A Router is Born
Widely considered the first router, BBN developed the Interface Message Processor (IMP) in the late 60's for the ARPANET. Bill Yeager developed a multiprotocol router at Stanford in 1980 while DEC was building "Fuzzballs," or routers supporting the internet's infancy with software developed by David Mills, the creator of Network Time Protocol (NTP) and Exterior Gateway Protocol (EGP). Later, Len Bosack and Sandy Lerner, who had been researchers at Stanford, founded Cisco and produced their first multiprotocol router Advanced Gateway Server (AGS) which shipped in 1986. In 1993 Cisco presented their first successful enterprise multiprotocol router, the 7000 series, and later would begin to supply switches as well.
X.25, Frame Relay and ATM merit a brief mention as these legacy protocols carry either SVC's or PVC's (Switched Virtual Circuits, Permanent Virtual Circuits) typically supplied by providers over purchased dedicated media for WAN connectivity. All are considered packet switching technologies, however X.25 provides SVC's, Frame Relay provides PVC's and ATM may supply either. Although no longer commonly used, in their day these protocols were at the forefront of WAN virtualization.
Interfaces derived from software that have no physical properties on their own are of course, virtual.
Loopbacks- There is a long history of loopbacks used in electronics, typically the routing of electronic signals back to the source for testing purposes. In networking the IETF made reference to the reserved address range "127.rrr.rrr.rrr" in 1981 with RFC 790 which also outlined 32 bit address space classes. In 1986 with RFC 990 the address range 127.rrr.rrr.rrr was officially dubbed "loopback". This number would lead to localhost assignment in computer networking, ie, it refers to the tcp/ip protocol stack of your device.
"The class A network number 127 is assigned the "loopback"
function, that is, a datagram sent by a higher level protocol
to a network 127 address should loop back inside the host. No
datagram "sent" to a network 127 address should ever appear on
any network anywhere."
The loopback would eventually be adopted by networking devices as an assignable/addressable virtual interface used for testing, routing, identification, filtering, and so on.
Other virtual interfaces include SVI's, null, tunnel, and subinterfaces, to name a few.
The acronym VPN, Virtual Private Network, describes a multitude of techniques used to accomplish a similar idea; a protected tunnel through publicly accessible internet media connecting two or more devices. This could take a variety of forms such as Site-to-Site VPN, SSL VPN,PPTP VPN, L2TP VPN, IPSEC VPN, MPLS VPN, etc.
VRF (Virtual Route Forwarding) is used to create separate and private ip routing tables within one or more routers. Consider the routing tables as individual and protected, accessible only by entities that are explicitly members of the routing table. Multiple routing tables are supported within a single router and can contain the same ip address scheme that will not overlap due to the division of routing table space. VRF is most often associated with MPLS, however Cisco has an implementation called VRF-Lite that functions without the need for Multi-Protocol Label Switching. Most manufacturers now ship devices with an interface vrf built in to provide dedicated OOB (Out-of-band) management, keeping the management traffic distinctly separate from the Control and Data Planes.
For switched networks, a Switched Virtual Interface (interface VLAN) is a method of assembling 1 or more layer 2 ports into a broadcast domain bounded by an ip address. This allows for layer 3 processing between switches with similarly configured port members, routing between VLANS, Gateway IP's for clients and layer 3 switch administration.
Combining a number of physical ports into a port-channel has the benefit of increasing the bandwidth of the resulting single logical interface by the sum of its members, limiting the impact of STP, improving redundancy and providing a means for load balancing. The link Aggregation Control Protocol ratified in 2000 by the IEEE as 802.3ad, the open standard, has since become the defacto standard.
Physically Two (or more), Logically One
With the various switch stacking techniques available today a number of switches may be grouped together with a single managed ip address thus making two or more fixed switches appear as a single network switch, as in a chassis switch with multiple line cards. Virtualization is carried yet further with techniques such as Virtual Switch System, whereby core switches act as one network element, reducing routing neighbors and providing a loop-free Layer 2 topology, as well as sharing control plane information and data traffic. VSS is a significant stride toward the elimination of Spanning-Tree in a redundant network.
High availability is often the goal with network virtualization techniques, aiming to increase operational performance, fault tolerance, reduce downtime and eliminate single points of failure. VRRP, HSRP and GLBP are First Hop Redundancy Protocols designed to keep client gateways less fallible. HA is often improved greatly with virtualization between like devices using active/standby, active/active, failover, and layer 2/layer3 table maintenance mechanisms via virtualized and aggregated links.
Network virtualization is accomplished with software logically simulating its hardware platform. "It is very close to being something, without actually being it." However, there is an idea near at hand-the complete decoupling of the control and data planes that will offer a centralized view of the network, affectionately referred to as Software Defined Networking.