Intra-Subnet Unicast Data Plane; Inter-Subnet Unicast Data Plane - Cisco ASR 9000 Series Configuration Manuallines

Intra-subnet Unicast Data Plane

Prefix Routing or No Subnet Stretch
IP reachability across the fabric is established using subnet prefix routes that are advertised using EVPN Route
Type 5 with the VPN label and VRF RTs. Host ARP and MAC sync are established across multi-homing
EVPN PEs using MAC+IP Route Type 2 based on a shared ESI to enable local switching through both the
multi-homing EVPN PEs.
Host Routing or Stretched Subnet
When a host is discovered through ARP, the MAC and IP Route Type 2 is advertised with both MAC VRF
and IP VRF router targets, and with VPN labels for both MAC-VRF and IP-VRF. Particularly, the VRF route
targets and Layer 3 VPN label are associated with Route Type 2 to achieve PE-PE IP routing identical to
traditional L3VPNs. A remote EVPN PE installs IP/32 entries directly in Layer 3 VRF table through the
advertising EVPN PE next-hop with the Layer 3 VPN label encapsulation, much like a Layer 3 VPN imposition
PE. This approach avoids the need to install separate adjacency rewrites for each remote host in a stretched
subnet. Instead, it inherits a key Layer 3 VPN scale benefit of being able to share a common forwarding rewrite
or load-balance resource across all IP host entries reachable through a set of EVPN PEs.
ARP and MAC sync
For hosts that are connected through LAG to more that one EVPN PE, the local host ARP and MAC entries
are learnt in data plane on either or both of the multihoming EVPN PEs. Local ARP and MAC entries are
synced across the two multihoming EVPN PEs using MAC and IP Route Type 2 based on a shared ESI to
enable local switching through both the multihoming EVPN PEs. Essentially, a MAC and IP Route Type 2
that is received with a local ESI causes the installation of a synced MAC entry that points to the local AC
port, and a synced ARP entry that is installed on the local BVI interface.
Note Only one Ethernet Flow Point (EFP) is supported per non-Zero ESI per bridge domain or EVI. This is a
limitation of EVPN.
MAC and IP Route Re-origination
MAC and IP Route Type 2 received with a local ESI, which is used to sync MAC and ARP entries, is also
re-originated from the router that installs a SYNC entry, if the host is not locally learnt and advertised based
on local learning. This route re-origination is required to establish overlay IP ECMP paths on remote EVPN
PEs, and to minimize traffic hit on local AC link failures, that can result in MAC and IP route withdraw in
the overlay.
Intra-subnet Unicast Data Plane
The Layer 2 traffic is bridged on the source EVPN PE using ECMP paths to remote EVPN PEs, established
through MAC+IP RT2, for every ES and for every EVI, ES and EAD Route Type 2 routes that are advertised
from the local EVPN PEs.

Inter-subnet Unicast Data Plane

Inter-subnet traffic is routed on the source EVPN PEs through overlay ECMP to the destination EVPN PEs
next-hops. Data packet are encapsulated with the VPN label advertised from the EVPN PE and tunnel label
for the BGP next-hop towards the spine. It is then routed again on the destination EVPN PE using a local
ARP adjacency towards the host. IP ECMP on the remote EVPN PEs is established through local and
re-originated routes advertised from the local EVPN PEs.
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
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Configure EVPN IRB