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No control plane is defined in the initial VxLAN solution. VxLAN tunnels are manually configured, and host addresses are learned through traffic flooding, which is highly undesirable with large Layer 2 domains. EVPN is used as a control plane for VxLAN to address this problem. It leverages MultiProtocol BGP (MP-BGP) to advertise information needed for the creation of VxLAN tunnels. In BGP-4, route updates are carried in Network Layer Reachability Information (NLRI) field. As BGP-4 can manage only IPv4 unicast routing information, MP-BGP was developed to support multiple network layer protocols, such as IPv6 and multicast. MP-BGP extends the NLRI field by adding the description of the address family to differentiate network layer protocols, such as IPv6 unicast address family and VPN instance address family. EVPN defines a new type of BGP NLRI called EVPN NLRI to enable learning of MAC addresses from other sites. EVPN NLRI defines the following types of routes:
Type 1 through Type 4 routes are defined in [RFC7432], while Type 5 routes are defined in [RFC9136]. This document focuses on the description of Type 2, Type 3, and Type 5 EVPN routes. Understanding these route types is required to understand the DCI operation. The Type 2 routes or Host (MAC/IP) advertisement routes are used by VTEPs to advertise the host IP and MAC address information to each other. VTEPs use Type 3 routes to advertise their IP addresses and L2VNIs to each other; that allows automatic VTEP discovery and dynamic creation of VxLAN tunnels. If there is a reachable route to the peer VTEP's IP address, a VxLAN tunnel is established from the local VTEP to the peer VTEP. Moreover, an ingress replication list is created for BUM packet forwarding, if the local and remote VNIs are the same. Lastly, Type 5 routes, are used to transmit network segment routes. Unlike Type 2 routes that transmit only 32-bit (IPv4) or 128-bit (IPv6) host routes, Type 5 routes can transmit network segment routes with mask lengths ranging from 0 to 32 or 0 to 128 bits. Using EVPN as a control plane of VxLAN has the following advantages: