4. BGP MPLS-Based EVPN Overview

4. BGP MPLS-Based EVPN Overview

This section provides an overview of EVPN. An EVPN instance comprises Customer Edge devices (CEs) that are connected to Provider Edge devices (PEs) that form the edge of the MPLS infrastructure. A CE may be a host, a router, or a switch. The PEs provide virtual Layer 2 bridged connectivity between the CEs. There may be multiple EVPN instances in the provider's network.

The PEs may be connected by an MPLS Label Switched Path (LSP) infrastructure, which provides the benefits of MPLS technology, such as fast reroute, resiliency, etc. The PEs may also be connected by an IP infrastructure, in which case IP/GRE (Generic Routing Encapsulation) tunneling or other IP tunneling can be used between the PEs. The detailed procedures in this document are specified only for MPLS LSPs as the tunneling technology. However, these procedures are designed to be extensible to IP tunneling as the Packet Switched Network (PSN) tunneling technology.

In an EVPN, MAC learning between PEs occurs not in the data plane (as happens with traditional bridging in VPLS [RFC4761] [RFC4762]) but in the control plane. Control-plane learning offers greater control over the MAC learning process, such as restricting who learns what, and the ability to apply policies. Furthermore, the control plane chosen for advertising MAC reachability information is multi-protocol (MP) BGP (similar to IP VPNs [RFC4364]). This provides flexibility and the ability to preserve the "virtualization" or isolation of groups of interacting agents (hosts, servers, virtual machines) from each other. In EVPN, PEs advertise the MAC addresses learned from the CEs that are connected to them, along with an MPLS label, to other PEs in the control plane using Multiprotocol BGP (MP-BGP). Control-plane learning enables load balancing of traffic to and from CEs that are multihomed to multiple PEs. This is in addition to load balancing across the MPLS core via multiple LSPs between the same pair of PEs. In other words, it allows CEs to connect to multiple active points of attachment. It also improves convergence times in the event of certain network failures.

However, learning between PEs and CEs is done by the method best suited to the CE: data-plane learning, IEEE 802.1x, the Link Layer Discovery Protocol (LLDP), IEEE 802.1aq, Address Resolution Protocol (ARP), management plane, or other protocols.

It is a local decision as to whether the Layer 2 forwarding table on a PE is populated with all the MAC destination addresses known to the control plane, or whether the PE implements a cache-based scheme. For instance, the MAC forwarding table may be populated only with the MAC destinations of the active flows transiting a specific PE.

The policy attributes of EVPN are very similar to those of IP-VPN. An EVPN instance requires a Route Distinguisher (RD) that is unique per MAC-VRF and one or more globally unique Route Targets (RTs). A CE attaches to a MAC-VRF on a PE, on an Ethernet interface that may be configured for one or more Ethernet tags, e.g., VLAN IDs. Some deployment scenarios guarantee uniqueness of VLAN IDs across EVPN instances: all points of attachment for a given EVPN instance use the same VLAN ID, and no other EVPN instance uses this VLAN ID. This document refers to this case as a "Unique VLAN EVPN" and describes simplified procedures to optimize for it.