PICOS version 4.0 and higher offer both Symmetric and Asymmetric routing for BGP EVPN hosts. In this document we will examine the case of Asymmetric EVPN routing with two hosts in different VNIs on two devices, R1 and R2. Figure 1 depicts how packet exchange between two hosts occurs in asymmetric BGP EVPN routing model. In the asymmetric routing model, the two end hosts, Host1 and Host2 are in two different VLANs and different VNIs. Host1 intends to communicate with Host2, sends a packet with destination MAC address of of R1 because R1 is configured as the gateway on Host1. On R1, the gateway is configured on an interface that belongs to VNI Y. The packet from Host1 on ingress at R1 is first routed to VNI Y within R1 and then bridged to R2 over the VXLAN tunnel VNI Y. On receiving the packet, R2 will strip off the VXLAN header and forward the packet to Host2.
Figure 1. EVPN Asymmetric Routing Model
The one limitation in this model is that both the devices must have the two VXLAN VNIs configured regardless if there are any hosts connected to it. In Figure 1, Host1 belongs to VNI X but both VNI X and VNI Y are configured on R1. Similarly, Host2 belongs to VNI Y but both VNI X and VNI Y are configured on R2 for asymmetric routing to work properly.