The node that performs IRB functionality in BGP EVPN is called the Layer3 gateway. EVPN standards outline various routing models and factors that influence the choice of routing model. Both Symmetric and Asymmetric IRB are part of the distributed routing model where each VTEP acts as a Layer3 gateway for the end host entities that are connected to it.
The Asymmetric IRB performs routing only on the ingress VTEP, while Symmetric IRB performs routing on both ingress and egress VTEPs. The topology below is used to explain the Symmetric IRB packet flow in the data plane.
In Symmetric IRB (Figure 1), Host H1 (VLAN 100) sends packets to Host H3 (different VLAN 200) and must be routed between the VLANs (and hence VNI). Host H1 packets arriving at a VXLAN access port at VTEP1 with IP destination IP 18.104.22.168 destined towards a virtual network subnet present in a Remote VTEP2 (VL200, VN 20000) is routed in the tenant VRF (IP‐VRF VNI 50000), VXLAN encapsulated, and is bridged over the IP‐VRF VNI 50000 towards the Remote VTEP2 where the destination virtual network subnet/host (host H3) is present. Once the packet reaches the Destination VTEP, it is decapsulated and is routed again to the destination subnet towards the destination host. Symmetric IRB is supported for tenant virtual network subnets over nondefault VRF only.
Figure 1. Symmetric IRB