EVPN E-Tree for MPLS and VXLAN

As part of an L2 EVPN service, E-Tree assigns the role of root or leaf to each attachment circuit (AC). After assigning the role, E-Tree enforces the following rules:
  • Root ACs can communicate with Leaf ACs and other Root ACs.
  • Leaf ACs can only communicate with Root ACs, and traffic between Leaf ACs blocked.

Configure the ACs at the VLAN level and enforce forwarding rules using a combination of locally configured leaf VLANs for local hosts and asymmetric route targets for remote hosts.

E-Tree supports the E-Tree extended community for EVPN Type-2 and Type-3 routes and allows E-Tree operation with only a single route-target configured, a regular non-E-Tree EVPN deployment. This support also allows EVPN MAC mobility to perform correctly in an E-Tree environment. Only deployments that use VXLAN encapsulation support the E-Tree extended community, and deployments using MPLS encapsulation must use asymmetric route targets.

When implementing a single route-target E-Tree, EOS supports configurable handling of remote leaf hosts in local leaf VLANs. By default, the local FDB for local leaf VLANs does not install remote local leaf hosts. Instead, a per-VLAN configuration allows the installation of explicit drop routes for remote leaf hosts, which removes unnecessary BUM traffic at the cost of local FDB space.

E-Tree Extended Community

The E-Tree extended community automatically attaches to all EVPN type-2 and type-3 routes advertised by a device associated with a VLAN locally configured as an E-Tree leaf AC. All type-2 routes that represent hosts learned on a leaf VLAN advertise the E-Tree extended community with a leaf flag set attached. All type-3 routes that represent a VLAN configured as a leaf AC advertise the E-Tree extended community with the leaf flag set attached.

After receiving an EVPN type-2 route with the E-Tree extended community flag set attached, a device checks the E-Tree role of the local VLAN to import the routes. If configured as an E-Tree root AC, the local floodlist for the VLAN adds the remote VTEP contained in the type-3 route. If the local VLAN has a configuration for an E-Tree leaf, the local floodlist filters the remote VTEP contained in the type-3 route. Leaf VLANS contain only remote VTEPs in the floodlist associated with the VLAN on the remote VTEP configured as an E-Tree root and prevents local leaf BUM traffic from flooding to the remote leaf ACs.

EVPN Active/Active Multihoming

EOS only supports EVPN Active/Active Multihoming on root VLANs. Only the peer that sees the host traffic learns the multihomed hosts if configured on leaf VLANs and using asymmetric route-target E-Tree. Because type-2 routes from other peers in the leaf VLANs do not import, an aliasing entry never installs for the hosts on the same Ethernet segment learned on other peers.

In addition, if configured on leaf VLANS, when using single route target extended community-based E-Tree, other peers in leaf VLANs import type-2 routes and associated aliasing entries do not install on the local FDB and treat them as remote leaf hosts.

In both cases, this leads to extra flooding as traffic destined for multihomed hosts that land on peers have not locally learned to treat the host as BUM traffic and floods.