Virtual Extensible LAN (VXLAN)

In this lesson, we will focus on one of the most important datacenter virtualization technologies, VXLAN (Virtual Extensible LAN). We will learn what is VXLAN, how VXLAN works and the other details of this virtualization or tunneling technology like VNI, VTEP, VXLAN Packet Format etc.

Then, let’s start with what is VXLAN question.



What is VXLAN?

VXLAN is the abbreviation of Virtual Extensible LAN. It is a network virtualization technology developed by Internet Engineering Task Force (IETF)Virtual Extensible LAN is an extension to classic VLANs (Virtual Local Area Network). It is basically a multi-point tunnelling technology. Virtual Extensible LAN basically encapsulates a layer 2 Ethernet frame into UDP packet and after that it transmit it over layer 3 network. As you see, here, layer 2 and layer 3 technologies are used together. As a summary, VXLAN is the tunneling technology which allows overlaying a Layer 2 network over a Layer 3 underlay with use of any IP routing protocol.


For Virtual Extensible LANs, Cisco Nexus 7000 switches are very good devices. Cisco Nexus 7000 switches provides hardware-based Virtual Extensible LAN function. With Cisco Nexus 7000 switches you can connect Virtual Extensible LAN and classic VLANs to work together.


After learning what is VXLAN, now let ‘s focus on the benefits of VXLAN.


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Benefits of VXLAN

Virtual Extensible LAN is an important datacenter virtualization technology in today’s networks. Virtual Extensible LAN provides important benefits to networking. So, what are these VXLAN benefits?


First of all, this virtualization technology expands our layer 2 network. This provides a flexible layer 2 for our networks.


Secondly, Virtual Extensible LAN technology increases scalability. It provides 16 million VXLAN segments at the same domain. This was not possible with previous VLAN technology. VLAN is limited if you compare with Virtual Extensible LAN technology. Because with 12-bit VLAN ID, we can create maximum 4000 VLANs.


With VXLAN, Layer 2 STP limitations have been overcome. ECMP and link aggregation technologies used in layer 3 help Virtual Extensible LAN to do this.


Another advantage of Virtual Extensible LAN is in VM Mobility. Virtual Extensible LAN provides seamless VM mobility.


With Virtual Extensible LAN, multicast is used instead of flooding broadcast and unknown unicast behavior.



How VXLAN Works?

With Virtual Extensible LAN, we can create virtual layer 2 subnets that span over layer 3 networks. Here, each layer 2 subnet is identified by VXLAN Network Identifier (VNI). To do this Virtual Extensible LAN technology uses encapsulation and decapsulation. Here, layer 2 ethernet frames are encapsulated in Layer 3 UDP packets by source VTEP (VXLAN Tunnel End Point). And it is also decapsulated at the destination VTEP.


To understand this mechanism better, let’s focus on some common terms used with Virtual Extensible LAN. These terms are VXLAN network identifier (VNI) and VTEP (Virtual Tunnel Endpoint). After that we will also learn the packet format of Virtual Extensible LAN.



VXLAN Network Identifier (VNI)

In VLANs, we use VLAN IDs. A VLAN ID represents every VLAN in the network. VLAN ID is 4 bytes. With this VLAN ID, we can create up to 4000 VLANs. This is a limited number for today’s networks.

With Virtual Extensible LAN, we use VXLAN network identifier (VNI). VNIs are similar to VLAN IDs. VNI is the value which identifies the virtual network in the data plane. Here, the total VXLAN header is 8 bytes and VNI is 3 bytes (24 bits) long. So, with VNI, we can create up to 16 million VXLAN. This is many more value if we compare it with VLANs.


There are two scopes of VNI. These are given below:

  • Network-wide scoped VNIs
  • Locally assigned VNIs


With Network-wide scoped VNIs, the same value is used to identify the specific Layer 3 virtual network across all network edge devices.


With Locally assigned VNIs, the identifier has local significance to the network edge device that advertises the route.



Virtual Tunnel End Point (VTEP)

VTEP (Virtual Tunnel Endpoint) is an edge device on a Virtual Extensible LAN network. It is the device that encapsulation and the decapsulation is done in Virtual Extensible LAN technology. In other words, it is the start and end point for VXVLAN Tunnel. A VTEP can be both a physical server or a software.


The source VTEP encapsulates the frames coming from source server, as VXLAN packets and sends them to the destination VTEP. The destination VTEP decapsulates these Virtual Extensible LAN packets and forwards it to the destination server.


On Virtual Tunnel Endpoint (VTE), there are two interfaces. These interfaces are:

  • IP Interface
  • Local LAN Interface


IP Interface is the interface towards the IP transport network and Local LAN Interface is the interface towards the local subnet.


VXLAN Packet Format

Encapsulation and decapsulation is done on VTEP (Virtual Tunnel Endpoint) as we have talked about before. Here, encapsulation is done over original layer 2 header.


Virtual Extensible LAN header is 8 bytes long. There are different parts of this header. These parts are given below:

  • VXLAN Flags
  • Reserved part
  • VNI
  • Reserved part

Here, VXLAN flags are 8 bits and the VXLAN network identifier (VNI) part is 24 bits. The first reserved part is 24 bits and the other reserved part is 8 bits.


At UDP header, there are UDP header fields as we have discussed in UDP lesson. Here, the important point is UDP destination port number. UDP destination port number is UDP 4789 port for VXLAN by default. UDP source port is calculated according to the ethernet header hash algorithm.


At Outer IP Header, there are source and destination IP addresses. Here, source IP address is VTEP IP address connected to the source VM. The destination IP address is VTEP IP address connected to the destination VM.


Outer MAC Header has source and destination MAC addresses. Here, source MAC address is the MAC address of VTEP connected to the source VM and the destination MAC address is the MAC address of the next hop along the path to the destination VTEP.



What is Virtual Extensible LAN Default Port?

The standard default port of VXLAN is UDP 4789. This is assigned by IANA (Internet Assigned Numbers Authority). But here there is an exception.


Virtual Extensible LAN was used in Linux firstly. At that time, Linux distributions was using UDP 8472 port for VXLAN. And for years many vendors used this port for Virtual Extensible LAN. After a while IANA has assigned UDP port 4789 to VXLAN. But at that time, there were many vendors that was using the previous port. So, Linux leave this default port UDP 8472. Now, Linux use UDP 8472 port for VXLAN while the other network equipment use UDP port 4789.



Cisco Virtual Extensible LAN Control And Data Plane

On Cisco devices, there are four different Virtual Extensible LAN Control and Data plane implementation are supported. These VXLAN Control and Data Plane implementations are given below:

  • With Multicast underlay
  • With static unicast VXLAN tunnels
  • With MP-BGP EVPN control plane
  • With LISP control plane


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