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Analysis of VXLAN Technology-(1) A brief introduction of VXLAN

Brief introduction of VXLAN

VXLAN,Virtual Extensible LAN, as its name implies, is an extended version of VLAN. VXLAN technology is mainly used to enhance the scalability of the network in cloud computing environment.

VXLAN uses UDP messages to encapsulate Ethernet messages to achieve data transmission across IP, and the UDP port number is 4789. The end device of VXLAN is called VTEP,VXLAN tunnel endpoint, which is generally borne by the port of physical switch (Nexus 5600) or virtual switch (Open vSwitch/Nexus 1000V). It is responsible for the encapsulation and unpacking of VXLAN messages.

VLAN provides logical layer 2 boundaries and broadcast domains. Compared with traditional VLAN, VXLAN extends the following features:

1. Provides a two-layer connectivity across three layers, which is popularly known as a two-layer tunnel.

two。 The use of 24bit's VNID greatly expands the two-tier logic domain provided by the original 12bit VLAN ID. 24bit means up to more than 16 million identifiers that are distinct from each other.

3. Use ECMP routing instead of STP to make better use of link bandwidth within the data center Fabric.

VXLAN was first proposed by VMware and Cisco, and has received contributions and support from other manufacturers, such as Arista, Broadcom, Citrix, Intel and Redhat. It is now standardized as IETF RFC7348.

VXLAN message format

VXLAN is a technology that encapsulates layer 2 data frames in UDP messages. VXLAN encapsulates the original layer 2 frame, plus the VXLAN header, in the UDP protocol to cross the layer 3 network.

VXLAN introduces an 8-byte VXLAN header that contains 24bit's VNID and some reserved bits. With the UDP/IP/Ethernet header, the overhead is 50 bytes more than the original Ethernet frame.

The detailed message format is shown in the following figure:

VXLAN packet encapsulation and de-encapsulation path (taking unicast as an example)

VXLAN uses stateless tunneling between VTEPs to transmit layer 2 data in a layer 3 network. A unicast VXLAN packet encapsulation and unencapsulation path is shown in the following figure:

As shown in the figure, Host-An and Host-B communicate with each other through a tunnel established on VTEP-1 and VTEP-2, the VXLAN subnet 10. This case assumes that address learning on both sides has been completed and that there is a corresponding MAC-to-VTEP mapping on the VTEP on both sides.

Comparison between OTV and VXLAN

If you have had previous contact with OTV, you may question the application scenario between the two. As a two-tier extension technology, what is the difference between the two? The following is a personal understanding, for reference only.

Overlay Transport Virtualization and Virtual Extensible LAN, you can see from the name that the focus between the two is essentially different, one focuses on the tunnel function, and the other is based on an extension to the existing LAN.

The original intention of OTV design is to get through the second layer of DCI, start from the control level, pay attention to routing, and use arp cache to restrain broadcast traffic. The original purpose of VXLAN design is to get through layer 2 in DC. At the beginning, it is to solve the problem at the data level, using layer 3 multicast to deal with BUM (Broadcast, Unknown Unicast, and Multicast) traffic. With the improvement of the VXLAN control plane (MP-BGP E × ×), VXLAN design across DCI will also become an option.

The data transmission message of OTV is encapsulated as Ethernet over MPLS over GRE, which is essentially a tunneling technology. VXLAN uses MAC in UDP encapsulation and encapsulates the VXLAN header in the UDP protocol. VNID in the VXLAN header expands the capabilities of VLAN ID and natively supports the cloud requirements of multi-tenancy.

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