External BGP peer receives large MPLS L3 VPN (VPNv4) table from OpenSDN

Note

This section covers MPLS L3 VPN (VPNv4 / VPN-IPv4), for example, Cisco show bgp vpnv4 unicast all or Juniper bgp.l3vpn.0. EVPN (BGP family l2vpn-evpn) peering and route advertisement are a different BGP family and are not subject of this section.

After establishing eBGP MPLS L3 VPN (VPNv4 / VPN-IPv4, BGP inet-vpn) peering between a data-center border router (for example, a Cisco ASR) and OpenSDN network or control nodes, the border may receive a large VPNv4 routing table containing many RD:IPv4-prefix entries and route targets for virtual-network routing instances.

This is expected control-plane behavior in common MP-BGP MPLS L3 VPN designs. OpenSDN controllers and network nodes distribute MPLS L3 VPN routes for tenant virtual networks; the border learns those VPNv4 NLRI entries without necessarily being able to forward into every tenant overlay. Internal routes may appear with non-usable next hops, for example, no-forward, on the OpenSDN side.

Virtual-network routing policies control inter-virtual-network leaks and related MP-BGP paths inside OpenSDN. They do not provide selective export from the fabric MPLS L3 VPN RIB to an external eBGP neighbor comparable to PE export filtering on a traditional edge router.

Planning guidance:

  1. If the design requires limiting which MPLS L3 VPN prefixes the border accepts or uses, configure import, route-target, or prefix filtering on the border router (or on intermediate route reflectors). Do not expect OpenSDN virtual-network routing policy to restrict VPNv4 advertisement on the inet-vpn session.

  2. Review External BGP MPLS L3 VPN (VPNv4) route advertisement for terminology, visibility versus forwarding, and border design notes.

  3. Configure ASN and encapsulation consistently with OpenSDN. Refer to Autonomous System Number (ASN) and Traffic encapsulation for details.