Symmetric IRB

Symmetric IRB uses the same Layer-3 VNI for bi-directional traffic between two hosts on different VLANs/VNIs. For example:

Figure 1  Symmetric IRB Topology


  1. Host1 in VLAN 10/VNI 10 connected to VTEP1 sends traffic to Host2 in VLAN 20/VNI 20 connected to VTEP2.
    1. Traffic from Host1 is sent to VTEP1 VLAN 10/ VNI 10 gateway MAC.
  2. VTEP1 routes traffic in the VRF (mapping to L3-VNI 1020), encapsulates traffic with VXLAN, adds outer Source/Destination IP, VNI info and sends traffic to VTEP2.
    1. VTEP1 does not need to have a MAC/ARP entry for Host2.
    2. VTEP1 learns the prefix route and/or host route corresponding to Host2 via EVPN Route type 5 or 2 respectively thus facilitating routing to the destination.
    3. The inner Source MAC is changed to VTEP1 router MAC and inner Destination MAC is changed to VTEP2 router MAC.
  3. VTEP2 decapsulates outer VXLAN, routes and sends the traffic to Host2 on VLAN 20/VNI 20.
    1. Host2 will see source MAC as VTEP2 gateway MAC.

Return traffic from Host2 to Host1 is similar, traffic from Host2 is sent to VTEP2 VLAN20/VNI 20 gateway MAC. VTEP2 routes traffic to L3 VNI 1020, encapsulates and sends the traffic to VTEP1. VTEP1 decapsulates, route and send the traffic to Host1 on VLAN 10/VNI 10.

As seen from the traffic flow:

  • Routing is used on both ingress and egress VTEPs.
  • Bi-directional traffic uses symmetric path; same L3 VNI in both directions.
  • VTEPs do not need to hold unnecessary ARP/MAC resources.
  • Destination VLAN/VNI does not have to be configured on source VTEP.