Arista EVPN VXLAN Configuration Example (3b) - Single-homing, L3 EVPN, Asymmetric IRB

To solve the sub-optimal routing pattern in the solution of centralized routing, the IRB EVPN Draft proposes 2 solutions, asymmetric IRB and symmetric IRB. Because the local VTEP does both inter-VLAN routing and intra-VLAN switching, it is called IRB (Integrated Routing and Bridging).

The asymmetric IRB is illustrated as below



Explanations:

• VTEP on has 1 directly connected VLAN:
• VTEP1 - VLAN 641
• VTEP2 - VLAN 642
• But the VTEPs must have
• 2 x SVI interface, VLAN 641 and 642
• 2 x VLANs under MAC VRF
• 2 x VLAN/VNI bindings under Vxlan interfce
• Routing is performed on the ingress VTEP, and egress VTEP only decapsulates the Vxlan header and switches into destination VLANs. 
• The returning traffic does the same, so routing is done on different VTEPs, hence the term Asymmetric IRB
• Advantage:
• Optimal traffic path and no traffic trombone 
• Disadvantage:
• VTEPs must have all SVIs and VLANs configured, even not locally connected. 
• That means ALL VTEPs hold ALL MAC and ARP of hosts for source and destination VLANs. 
• So the scale is the biggest issue. To make things worse, TOR devices normally don't much high capacity.  

From the below output, the VTEP1 has 6 ARP entries, 3 local VLANs and 3 remote VLANs

snp261-eVtep1.23:11:00#sh arp

Address         Age (sec)  Hardware Addr   Interface

160.64.1.101      0:02:54  444c.a8a5.1140  Vlan641, Ethernet78

160.64.1.102      0:01:08  444c.a8a5.1140  Vlan641, Ethernet78

160.64.1.103      0:01:04  444c.a8a5.1140  Vlan641, Ethernet78

160.64.2.201            -  444c.a8a5.1141  Vlan642, Vxlan1

160.64.2.202            -  444c.a8a5.1141  Vlan642, Vxlan1

160.64.2.203            -  444c.a8a5.1141  Vlan642, Vxlan1