What is a Virtual Extensible LAN?

July 8th, 2016
What is a Virtual Extensible LAN?

Virtual Extensible LAN (VXLAN) is a LAN segmentation and overlay solution. VXLAN is designed to provide up to 16 million layer 2 segments. It can be utilized to extend a layer 2 segment over underlying layer 3 network infrastructure. VXLAN uses 24-bits to identify a network segment, which is known as VXLAN network identifier. 

VXLAN utilizes MAC-in-UDP encapsulation to provide connectivity across a layer 2 segment located in different parts of a network. This is accomplished by encapsulating the original layer two frame into VXLAN UDP-IP header. The VXLAN header consists of 20 bytes of IP header, 8 bytes of UDP header, and 8 bytes of VXLAN header.

The connectivity across VXLAN segments is provided by VXLAN tunnel endpoint(VTEP) devices. The source VTEP encapsulates layers 2 frames from endpoints on its local VXLAN segment, and transports them over the underlying IP network to far-end VXLAN segment. The far-end VTEP de-encapsulates the packet and forwards the layer two frame based on the source VXLAN Network Identifier. 

IP multicast is used to transport layer 2 broadcast, unknown unicast, and multicast traffic from end-points in VXLAN. To avoid unnecessary replication of layer two traffic, it is recommended to use one-to-one mapping of VXLAN network identifier to multicast group address. VXLAN-toVLAN connectivity is provided by VXLAN gateway, which combines VXLAN segment and classical VLAN segment into one common layer 2 domain.

VXLAN Tunnel Endpoint and VXLAN gateway features are supported in hardware on most Cisco Nexus 9000 platforms. Detailed operational behavior of VXLAN is covered in IETF RFC 7348.


Conclusion:
VXLAN provides standardized method to extend layer 2 segment across layer 3 network infrastructure, enabling flexible workload placements by way of layer two extension. 

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