Local and Remote End-Host Learning

The MP-BGP EVPN control plane provides integrated routing and bridging by distributing both Layer 2 and Layer 3 reachability information for the end host residing in the VXLAN overlay networks. Each VTEP performs local learning to obtain MAC and IP address information from its locally attached hosts and then distributes this information through the MP-BGP EVPN control plane. Hosts attached to remote VTEPs are learned remotely through the MP-BGP control plane. This approach reduces network flooding for end-host learning and provides better control over end-host reachability information distribution.

A VTEP in MP-BGP EVPN learns the MAC addresses and IP addresses of locally attached end hosts through local learning. This learning can be local data plane based using the standard Ethernet and IP learning procedures, such as source MAC address learning from the incoming Ethernet frames and IP address learning when the hosts send Gratuitous ARP (GARP) and Reverse ARP (RARP) packets or ARP requests for the gateway IP address on the VTEP.

After learning the localhost MAC and IP addresses, a VTEP advertises the host information in the MP-BGP EVPN control plane so that this information can be distributed to other VTEPs. This approach enables EVPN VTEPs to learn the remote end hosts in the MP-BGP EVPN control plane. The EVPN routes are advertised through the L2VPN EVPN address family. The BGP L2VPN EVPN routes include the following information:

• Route distinguisher (RD): A route distinguisher is an 8-bit octet number used to distinguish one set of routes (one VRF instance) from another. It is a unique number prepended to each route so that if the same route is used in several different VRF instances, BGP can treat them as distinct routes. The route distinguisher is transmitted along with the route through MP-BGP when EVPN routes are exchanged with MP-BGP peers.
• MAC address length: 6 bytes
• MAC address: Host MAC address
• IP address length: 32 or 128
• IP address: Host IP address (IPv4 or IPv6)
• L2 VNI: VNI of the bridge domain to which the end host belongs
• L3 VNI: VNI associated with the tenant VRF routing instance

MP-BGP EVPN uses the BGP extended community attribute to transmit the exported route targets in an EVPN route. When an EVPN VTEP receives an EVPN route, it compares the route target attributes in the received route to its locally configured route target import policy to decide whether to import or ignore the route. This approach uses the decade-old MP-BGP VPN technology (RFC 4364) and provides scalable multitenancy in which a node that does not have a VRF locally does not import the corresponding routes. Route target is an extended-community attribute to filter appropriate VPN routes into the correct VRFs.

When a VTEP switch originates MP-BGP EVPN routes for its locally learned end hosts, it uses its own VTEP address as the BGP next hop. This BGP next hop must remain unchanged through the route distribution across the network because the remote VTEP must learn the originating VTEP address as the next hop for VXLAN encapsulation when forwarding packets for the overlay network.

Figure 7-13 shows local and remote end-host address learning and distribution in an MP-iBGP EVPN using route reflectors.

  

Figure 7-13 Local and Remote Host Address Learning In Figure 7-13, VTEP-1 learns the MAC addresses and IP addresses of locally attached end hosts through local learning. VTEP-1 then sends a BGP update to the route-reflector in the transit network, informing about the host IP (H-IP-1) and MAC (H-MAC-1) address along with the L2-VNI information. The next-hop in the MP-BGP EVPN route update is set to VTEP-1. When remote VTEP-2 and VTEP-3 receive the route update from the route reflector, they install the host information in their routing information base (RIB) and forwarding information base (FIB).