VXLAN DATA PLANE

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The VXLAN data plane is responsible for forwarding data over VXLAN tunnels after address mappings are learned on the control plane. A source VTEP adds a UDP header to the original data frame, which is later removed by the destination VTEP. The intermediate network devices forward the packet based on the destination address in the outer IP header. Data forwarding scenarios include intrasubnet known unicast packet forwarding, intrasubnet BUM packet forwarding, and intersubnet packet forwarding.

Intrasubnet Packet Forwarding

1. Intrasubnet forwarding of known unicast packets

Known unicast packets for intrasubnet communication are forwarded only between Layer 2 VXLAN gateways. Layer 3 VXLAN gateways are unaware of this process. Figure 5.17 shows the packet forwarding process.

a. When receiving a packet from Host 3, VTEP 2 determines the Layer 2 BD of the packet based on the access interface and VLAN information in the packet, and searches for the outbound interface and encapsulation information in the Layer 2 BD.

b. VTEP 2 performs VXLAN encapsulation based on the encapsulation information and forwards the packet through the outbound interface.

c. Upon receipt of the VXLAN packet, VTEP 3 verifies the VXLAN packet based on the UDP destination port number, source and destination IP addresses, and VNI. Then, VTEP 3 obtains the Layer 2 BD based on the VNI and performs VXLAN decapsulation to obtain the inner Layer 2 packet.

d. Once VTEP 3 obtains the destination MAC address of the inner Layer 2 packet, it processes VLAN tags for the packet based on the outbound interface and encapsulation information in the local MAC address table, and forwards the packet to Host 2.

Host 2 sends packets to Host 3 in the same manner.

Intrasubnet forwarding of known unicast packets

FIGURE 5.17 Intrasubnet forwarding of known unicast packets.

2. Intrasubnet forwarding of BUM packets

Intra-subnet BUM packets are forwarded in ingress replication mode between Layer 2 VXLAN gateways. Layer 3 VXLAN gateways are unaware of this process.

In ingress replication mode, after a BUM packet enters a VXLAN tunnel, the ingress VTEP performs VXLAN encapsulation based on the ingress replication list and forwards the packet to all egress VTEPs in the list. Then, when the BUM packet leaves the VXLAN tunnel, the egress VTEPs decapsulate the BUM packet. Figure 5.18 shows the forwarding process of a BUM packet in ingress replication mode.

Intrasubnet forwarding process of a BUM packet in ingress replication mode

FIGURE 5.18 Intrasubnet forwarding process of a BUM packet in ingress replication mode.

a. After receiving a packet from Terminal A, VTEP 1 determines the Layer 2 BD of the packet based on the access interface and VLAN information in the packet.

b. VTEP 1 obtains the ingress replication list for the VNI based on the Layer 2 BD, replicates the packet based on the list, and performs VXLAN encapsulation on the packet. VTEP 1 then forwards the VXLAN packet through the outbound interface.

c. Upon receipt of the VXLAN packet, either VTEP 2 or VTEP 3 verifies the VXLAN packet based on the UDP destination port number, source and destination IP addresses, and VNI. VTEP 2 or VTEP 3 obtains the Layer 2 BD based on the VNI and perform VXLAN decapsulation to obtain the inner Layer 2 packet.

d. VTEP 2 or VTEP 3 then checks the destination MAC address of the inner Layer 2 packet and finds it a BUM MAC address. Therefore, VTEP 2 or VTEP 3 broadcasts the packet on the network connected to terminals (user side) in the Layer 2 BD. Specifically, VTEP 2 or VTEP 3 finds the outbound interfaces and encapsulation information not related to the VXLAN tunnel from the local MAC address table, processes VLAN tags for the packet, and forwards the packet to either Terminal В or Terminal C.

Intersubnet Packet Forwarding

Intersubnet packets must be forwarded through a Layer 3 gateway.

Figure 5.19 shows the intersubnet packet forwarding process in a centralized VXLAN gateway scenario.

  • 1. After receiving a packet from Host 1, VTEP 2 determines the Layer 2 BD of the packet based on the access interface and VLAN information in the packet, and searches for the outbound interface and encapsulation information in the BD.
  • 2. VTEP 2 performs VXLAN encapsulation based on the outbound interface and encapsulation information and forwards the VXLAN packet to VTEP 1.
  • 3. After receiving the VXLAN packet, VTEP 1 decapsulates the packet and finds that the destination MAC address of the inner packet is the MAC address (MAC 3) of the Layer 3 gateway interface (VBDIF 10). In this case, the packet must be forwarded at Layer 3.
  • 4. VTEP 1 removes the inner Ethernet header, parses the destination IP address, and searches the routing table for the next-hop address. Then, VTEP 1 searches the ARP table based on the next-hop address
Intersubnet packet forwarding

FIGURE 5.19 Intersubnet packet forwarding.

to obtain the destination MAC address, VXLAN tunnel’s outbound interface, and VNI.

5. VTEP 1 performs VXLAN encapsulation on the inner packet again and forwards the VXLAN packet to VTEP 3, with the source MAC address in the inner Ethernet header being the MAC address (MAC 4) of the Layer 3 gateway interface (VBDIF 20).

  • 6. Upon receipt of the VXLAN packet, VTEP 3 verifies the VXLAN packet based on the UDP destination port number, source and destination IP addresses, and VNI. VTEP 3 then obtains the Layer 2 BD based on the VNI and removes the outer headers to obtain the inner Layer 2 packet. It then searches for the outbound interface and encapsulation information in the Layer 2 BD.
  • 7. VTEP 3 processes VLAN tags for the packet based on the outbound interface and encapsulation information and forwards the packet to Host 2.

Host 2 sends packets to Host 1 in the same manner.
 
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