Control plane behavior

From a control-plane perspective, an egress leaf working in Anycast MH mode (such as Leaf1 and Leaf2 in EVPN Anycast MH) for a given ES works as follows:

  • AD per-EVI routes for Anycast MH ESes are suppressed based on the Anycast MH mode configuration.
    • AD per-EVI routes are suppressed only for MAC-VRFs with VXLAN encapsulation.
    • This applies to nodes sharing ESs by mac-vrfs with vxlan and mpls encap.
  • AD per-ES routes for Anycast MH ESes are advertised with the A=1 flag in ESI label extended community (as defined in draft-rabnag-bess-evpn-anycast-aliasing).
    • The anycast-multi-homing flag indicates to remote NVEs that EVPN MAC/IP Advertisement routes with matching Ethernet Segment Identifier are resolved only by A-D per ES routes for the ES. This flag signals to the ingress leaf that no A-D per EVI routes are advertised for the Ethernet Segment. The A flag is bit 2 in the Flags field of the ESI label extended community.
      Figure 1. ESI label extended community
    • The BGP tunnel-encapsulation attribute ( per draft-rabnag-bess-evpn-anycast-aliasing) carries the Anycast VTEP; specifically, it is encoded in the BGP Tunnel Encapsulation Attribute Tunnel Egress Endpoint Sub-TLV (code point 6) [RFC9012]. The Tunnel Egress Endpoint Sub-TLV Value Field is shown below.
      Figure 2. Tunnel egress endpoint Sub-TLV
  • MAC/IP routes for MACs learned on the ES working in Anycast MH mode are advertised with ESI. The next-hop is either the regular system0.0 IPv4 address or the one determined by network-instance[=mac-vrf].protocols.bgp-evpn.bgp-instance.routes.bridge-table.next-hop <ip-address>