BGP EVPN Route Types

To share routing information with its peers, BGP uses update packets. Routes with the same path attributes are placed in the Network Layer Reachability Information (NLRI) of the update packet and advertised. Since traditional BGP-4 only supports IPv4 unicast routing information, Multiprotocol Extensions for BGP (MP-BGP) was developed to provide additional support for network layer protocols such as multicast and IPv6. MP-BGP extensions are added to the NLRI after which description for different protocols like IPv6 unicast and VPN instance family are subsequently added.

EVPN defines the EVPN sub-address family in the L2VPN address family with the introduction of EVPN NLRI. Once the routes are advertised, VXLAN tunnels are automatically established to carry packets. EVPN NLRI has the following EVPN route types:

• Type-1, Ethernet Auto-Discovery Routes: are used for network wide messaging. The Ethernet auto discovery routes are used when a host or server is multihomed to the EVPN VXLAN fabric. It is used to signal to the upstream VTEPs to change their next hop adjacencies for all the MAC addresses associated with a particular ethernet segment in the event of a link failure. The Type-1 EAD routes has the following makeup shown in Table 1. The Ethernet Segment Identifier (ESI) has a non-zero value for multihomed hosts. In case when a device is single-homed, the value of Ethernet Subnet Identifier (ESI) is set to zero. Type-1 routes are advertised on per ESI and per EVI basis and used to achieve fast convergence.

Table 1. Ethernet Auto Discovery Route NLRI

• Type-2, MAC with IP advertisement Routes: are used to advertise the MAC and IP addresses of hosts. When a VTEP discovers a new host MAC/IP address, it will use Type-2 routes to share this new host with all the other concerned VTEPs in the EVPN VXLAN fabric. This behavior of BGP EVPN is also known as remote host learning and one of the key benefit of having a control plan for VXLAN. The Type-2 route NLRI has the following fields.

Table 2. EVPN MAC/IP Advertisement Route

• Type-3, Inclusive Multicast Routes: are used for the automatic discovery of VTEPs and dynamically establishing VXLAN tunnels. Specifically, Type-3 routes are used to deliver multicast, unknown unicast and broadcast traffic across the EVPN VXLAN fabric. When a VTEP receives a multicast packet encapsulated with a VLAN tag, the ingress VTEP will distribute the packet to all VTEPs that span this VLAN in the given EVPN instance.  Type-3 routes have the following NLRI fields.

Table 3. EVPN Type-3 Route NLRI

• Type-4, Ethernet Segment Routes: are needed in multihoming scenarios and used for Designated Forwarder Election. Designated Forwarder is responsible for sending broadcast, unknown unicast and multicast (BUM) traffic to devices on an Ethernet Segment. 

  The EVPN NLRI for Type-4 Ethernet Segment Rout consists of the following four fields. For multihomed hosts, the Ethernet Segment Identifier (ESI) must be set to a non-zero value. A value of zero for the ESI indicates a single homed device.

  
  

  RD (8 Octets)

  Ethernet Segment Identifier (10 Octets)

  IP Address Length (1 octet)

  Originating Router's IP Address (4 or 16 octets)

  Table 2. EVPN Type-4 NLRI

• Type-5, IP Prefix Route: provides encoding for inter-subnet forwarding. These routes are used for advertising IP prefixes for connectivity between different subnets across the enterprise. Type-5 routes can advertise both IPv4 and IPv6 prefixes. The NLRI fields of IPv4 is shown below.

Table 5. Type-5 IPv4 Route NLRI

The NLRI fields in an IPv6 address are given below.

Table 6. Type-5 IPv6 Route NLRI