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Network Packet Broker VXLAN Header Stripping VTEP Message Transfer via Multicast
The underlying network of vxlan is three-layer, and the broadcast address cannot cross the three-layer network, so all vtep messages sent to the vxlan network can only be multicast.
The address process of the opposite virtual machine MAC is as follows:
The virtual machine A at the bottom left should send messages to the virtual machine B at the bottom right through the vxlan network
1- A sends ARP request (IP of request B) in broadcast mode, and the source MAC is mac-a.
2- VTEP1 encapsulates ARP request as vxlan message, which has been multicast to multicast group 239.1.1.1.The source MAC is the MAC address mac-1 of VTEP1, the destination MAC is the multicast MAC address 00:01:5E:01:01:01, the source IP is the IP address ip-1 of VTEP1, and the destination IP is the multicast address 239.1.1.1.
3- After VTEP2(VTEP3) receives the vxlan message, the outer vxlan header is stripped off, and the broadcast ARP request is sent in the second layer network, and the triple of mac-a | VNI | ip-1 is learned.
4- After receiving the ARP request, B will send the ARP reply via unicast, with MAC address mac-b as the source MAC of B, and learn the ARP table ip-a | mac-a.
5- When VTEP2 receives the ARP reply from B, plus the vxlan header, VTEP2 already knows the information of virtual machine A and VTEP1, and sends out vxlan message by unicast. The destination IP is VTEP1's IP address ip-a, the source IP is its own IP address ip-b, and VNI is learning VNI 10.
6- VTEP1 receives the unicast vxlan message sent by VTEP2 and learns the mac-b | VNI | ip-2 triplet information of virtual machine B and VTEP2.
7- VTEP1 strips the vlxan header of the vxlan message and sends the ARP reply to a. when the virtual machine A gets the ARP reply, it will know the MAC address of the destination virtual machine B.
Distributed Control Center
In the virtual machine and container scenario, when the virtual machine or container has not yet started network communication, we can know its IP and MAC, the distributed control center keeps this information.In addition, the control center keeps track of what vteps are in each vxlan network and what the addresses of those vteps are.With this information, vtep can directly query and add headers when sending a message, without multicasting to the network.
In general, there will be an agent at the node of each vtep, which will communicate with the control center to obtain the information needed by vtep and tell vtep in some way.Concrete practice depends on the specific implementation, the implementation may update the different information to the vtep, such as its ehrs (Head End Replication) just have replaced the multicast group into multiple single broadcast, namely the multicast group all tell vtep vtep IP address, not sending multicast such queries, but for every vtep group sends a single broadcast wen;Some implementations simply tell the vtep destination virtual machine's MAC address information;Some implementations tell the MAC address the corresponding vtep IP address.
Also, it makes a difference when you tell vtep.
There are two ways:
Push: it is common to tell vtep once you know the triplet information of the virtual machine (even if a vtep cannot use this information, because the virtual machine it manages will not communicate with this address), usually at this time the first communication has not occurred;
Query: another way is to inform the agent in a certain way when the vtep needs the information in the first communication, and then the agent tells the vtep information at this time.
The Data Transfer
After the ARP request is answered, A knows B's MAC address and sends data to B.VTEP1 receives the packet of A, and checks whether A and B use the same VNI with MAC address (otherwise VXLAN gateway is required). VTEP1 already knows the information of B and VTEP2, directly carries out VXLAN packet encapsulation, and then delivers it to the upper connected device for transmission (UDP message).
Intermediate transmission process is the ordinary IP packet transmission process.
Objective VTEP checks the VNI after receiving the vxlan message. If it is consistent with the VNI of B, it will unpack the packet into the vxlan header and send it to B for processing.For A and B, it's like communicating within A local area network.
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