Adding TS’s IP Address to MAC-VRF (L2RIB) and IP-VRF (L3RIB)

In the previous chapter, we discussed how a VTEP learns the local TS's MAC address and the process through which the MAC address is programmed into BGP tables. An example VTEP device was configured with a Layer 2 VLAN and an EVPN Instance without deploying a VRF Context or VLAN routing interface. This chapter introduces, at a theoretical level, how the VTEP device, besides the TS's MAC address, learns the TS's IP address information after we have configured the VRF Context and routing interface for our example VLAN.

Figure 1-3: MAC-VRF Tenant System’s IP Address Propagation.

I have divided Figure 1-3 into three sections. The section on the top left, Integrated Routing and Bridging - IRB illustrates the components required for intra-tenant routing and their interdependencies. By configuring a Virtual Routing and Forwarding Context (VRF Context), we create a closed routing environment with a per-tenant IP-VRF L3 Routing Information Base (L3RIB). Within the VRF Context, we define the Layer 3 Virtual Network Identifier (L3VNI) along with the Route Distinguisher (RD) and Route Target (RT) values. The RD of the VRF Context enables the use of overlapping IP addresses across different tenants. Based on the RT value of the VRF Context, remote VTEP devices can import IP address information to the correct BGP tables, and from where they are installed into the IP-VRF's L3RIB. For the VRF Context, we configure a Layer 2 VLAN (VLAN 50 in the image), which we associate with the L3VNI. Besides, we create an IP address-less routing interface for the VLAN and bind it with the VRF Context. These configurations are necessary because VXLAN is MAC-in-IP/UDP and requires an inner Ethernet header with source/destination MAC addresses for routed inter-VN traffic.

By deploying a VLAN with its routing interface, besides reserving hardware resources, the VTEP device can use a system MAC address in the inner Ethernet header in routed packets. To enable Data Plane tunneling, we must associate the VRF Context with the NVE interface. After setting up a VRF Context with its components, we can attach VLANs requiring inter-VN or external connections to the VRF Context. The upcoming chapters show how we deploy the configuration and verify the Control Plane and Data Plane operation.

The section at the bottom left, MAC-VRF Update Process – IP Address, describes the process where the IP address is associated with the MAC address of TS in the EVPN Instance's MAC-VRF L2RIB.

When we power on the TS, it may send a Gratuitous ARP (GARP) message to ensure the uniqueness of its IP address. TS may also send an ARP request to resolve the IP-MAC binding of its defined Default Gateway. In our example, the VTEP switch receives the GARP message from TS1 through the Attachment Circuit (AC) Eth1/10. The Sender MAC and IP address information carried in the GARP message and the VLAN-specific routing interface (Gateway - GW) are encoded in the ARP table by the Host Mobility Manager (HMM). Note that there must be a routing interface on the VLAN for the MAC-IP bindings of devices connected to the VLAN to be stored in the ARP table.

Next, HMM programs the ARP table with MAC/IP/GW information and stores it in the Attachment Circuit's Local Host Database (LHDB). Then, HMM encodes MAC/IP/Next-Hop information into the MAC-VRF's L2RIB along with the VLAN Identifier and L3VNI (Layer 3 Virtual Network Identifier). In addition, HMM encodes the IP address information as a host route in the IP-VRF. 

The BGP process programs the information into the Loc-RIB. EVPN NLRI includes the Route Distinguisher (RD), MAC and IP address information, and corresponding L2VNI and L3VNI identifiers. Route Targets, Encapsulation Type, and Router MAC are encoded as Extended Path Attributes. Finally, the routing information about EVPN NLRI is sent through the BGP Policy Engine to the BGP Adj-RIB-Out table and eventually to BGP EVPN peers.