Traditional VLAN technology cannot meet the strict requirements of cloud providers because it can only support up to 4096 VLANs, but VXLAN technology can build large-scale multi-tenant data centers by extending Layer 2 to Layer 3 networks. Favored, this technology can effectively overcome the expansion limitations brought by VLAN.

What is vlan? ?

VLAN (Virtual Local Area Network) means virtual local area network, which is a concept involved in the implementation of switches and is defined by the 802.1Q standard. Since switches are network devices working at the link layer, terminals connected to the same switch are in the same Layer 3 network and are also in the same broadcast domain. When the switch is connected to many terminals, when any terminal sends a broadcast packet (for example: ARP request), the packet will be spread throughout the entire network.

Vlan is divided into static and dynamic. When the VLAN administrator initially configures the corresponding relationship between the switch port and the VLAN ID, the static vlan has already fixed the corresponding relationship, that is, the port can only correspond to this VLAN ID, and cannot be changed later unless the administrator reconfigures it. Dynamic vlan means that the switch automatically configures the port as the VLAN to which the host belongs. There are three classifications here: MAC-based, IP-based, and user-based.

For large-scale networking scenarios, the flood of broadcast packets will have a greater impact on network communications. VLAN technology provides a solution to this problem. VLAN divides the same network into multiple logical virtual subnets, and stipulates that when a broadcast packet is received, it is only broadcast in the VLAN where it is located to prevent the flood of broadcast packets. . VLAN technology implements the isolation of broadcast domains at the link layer level.

The development of virtualization technology has prompted big data and cloud computing technology companies to use a single physical device to virtualize multiple virtual machines for networking. Therefore, the requirements for the number of VLANs supported are also increasing. The technology supports up to 4094 VLANs, so it can no longer meet the demand.

What is Vxlan?

VXLAN (Virtual eXtensible Local Area Network, Virtual Extended Local Area Network) is one of the NVO3 (Network Virtualization over Layer 3) standard technologies defined by the IETF, using L2 over L4 (MAC-in-UDP) Packet encapsulation mode, which encapsulates Layer 2 packets with Layer 3 protocols, enables Layer 2 network expansion within Layer 3, and meets the needs of large Layer 2 virtual migration and multi-tenancy in data centers. VXLAN technology can build a layer-2 virtual network based on a layer-3 network structure. Through VLAN technology, network devices in different network segments can be integrated into the same logical link layer network. For end users, these network devices seem to be “real”. ” is deployed in the same link layer network.

NVO3 is a general term for technologies that build virtual networks based on three-layer IP overlay networks. VXLAN is just one of NVO3 technologies. In addition, the more representative ones are NVGRE and STT.

Compared with VLAN technology, VXLAN technology has the following advantages:

The 24-bit length of the VNI field value can support a larger number of virtual networks, solving the problem that the maximum number of VLANs is 4094. limitation issue.

VXLAN technology virtualizes a Layer 2 network in a physical Layer 3 network through tunneling technology. Terminals on the VXLAN network cannot perceive the VXLAN communication process, which makes the logical network topology and physical network topology realized. With a certain degree of decoupling, the configuration of the network topology is less dependent on the configuration of physical devices, and the configuration is more flexible and convenient.

VLAN technology only solves the problem of Layer 2 network broadcast domain segmentation, while VXLAN technology also has the feature of multi-tenant support. The issue of address conflicts between tenants has also been resolved.

In order to ensure the correctness of the communication process of the VXLAN mechanism, the rfc7348 standard stipulates that IP packets involving VXLAN communication are not allowed to be fragmented, which requires that the link layer implementation of the physical network must provide sufficient A large MTU value ensures the smooth transmission of VXLAN packets, which can be understood as a limitation of the current VXLAN technology.

Generally speaking, the default MTU of a virtual machine is 1500 Bytes, which means that the maximum size of the original Ethernet packet is 1500 bytes. When this message passes through VTEP, it will encapsulate a new 50-byte header (VXLAN header 8 bytes + UDP header 8 bytes + external IP header 20 bytes + external MAC header 14 bytes), so that , the entire message length reaches 1550 bytes. On the other hand, the existing VTEP equipment generally requires that the VXLAN packets cannot be fragmented when decapsulating VXLAN packets, otherwise the VXLAN packets cannot be decapsulated correctly. This requires a minimum MTU of 1550 bytes for all network devices between VTEPs.

If the MTU value of the intermediate device is inconvenient to change, setting the MTU value of the virtual machine to 1450 can also temporarily solve this problem.

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