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Network World - There are a number of emerging and proposed standard protocols focused on optimizing the support that data center Ethernet LANs provide for server virtualization. Several of these protocols are aimed at network virtualization via the creation of multiple virtual Ethernet networks that can share a common physical infrastructure in a manner that is somewhat analogous to multiple virtual machines sharing a common physical server.
Most protocols for network virtualization are based on creating virtual network overlays using techniques based on encapsulation and tunneling. The most commonly discussed protocols include VXLAN, NVGRE, STT, and SPB MAC-in-MAC. SPB is already an IEEE standard, while it is likely that only one of the other proposals will achieve IETF standard status, most likely VXLAN.
Traditional network virtualization
The one-to-many virtualization of network entities is not a new concept. The most common examples are VLANs and Virtual Routing and Forwarding (VRF) instances.
VLANs partition the network into as many as 4,094 broadcast domains, as designated by a 12-bit VLAN ID tag in the Ethernet header. VLANs have been a convenient means of isolating different types of traffic that share the same switched LAN infrastructure.
In data centers that make extensive use of server virtualization, the limited number of VLANs can present problems, especially when large number of tenants need to be supported, each requiring multiple VLANs. Extending VLANs across the data center via 802.1Q trunks to support VM mobility adds operational cost and complexity. In data centers based on Layer 2 server-to-server connectivity, large numbers of VMs, each with its own media access control address, can also place a burden on the forwarding tables capacities of Layer 2 switches.
VRF is a form of Layer 3 network virtualization in which a physical router supports multiple virtual router instances, each running its own routing protocol instance and maintaining its own forwarding table.
Unlike VLANs, VRF does not use a tag in the packet header to designate the specific VRF to which a packet belongs. The appropriate VRF is derived at each hop based both on the incoming interface and on information in the frame. An additional requirement is that each intermediate router on the end-to-end path followed by a packet needs to be configured with a VRF instance that can forward that packet.
Network Virtualization with Overlays
Because of the shortcomings of the traditional VLAN or VRF models, a number of new techniques for creating virtual networks have recently emerged. Most are based on the use of encapsulation and tunneling to construct multiple virtual network topologies overlaid on a common physical network.
A virtual network can be a Layer 2 network or a Layer 3 network, while the physical network can be Layer 2, Layer 3 or a combination depending on the overlay technology. With overlays, the outer (encapsulating) header includes a field that is generally 24 bits wide that carries a virtual network instance ID (VNID) that specifies the virtual network designated to forward the packet.