CNX Documentation

Add-Path and ECMP

Ordinary BGP sends one selected path for a prefix to a neighbor. Add-Path adds a path identifier to BGP updates so several paths for the same prefix can remain present on one session.

CNX route servers offer Add-Path in the transmit direction. A member router must advertise the corresponding receive capability before the route server can send additional paths. CNX also enables route-server path-hiding mitigation, allowing an eligible alternate path to be considered when policy prevents export of the first path.

Maintenance resilience

The three route-server sessions and Add-Path provide two different layers of resilience:

  • peering with all three route servers keeps the same eligible prefixes available through the other two sessions when CNX maintains one server; and
  • Add-Path keeps multiple participant paths for a prefix available within each route-server session instead of exposing only that server's single best path.

When the member router maintains all six IPv4 and IPv6 sessions, applies consistent policy, retains alternate paths, and keeps usable next hops in its routing or forwarding table, CNX can rotate one route server out of service without interrupting forwarding. Add-Path reduces the chance that withdrawal of one control-plane path must be followed by learning and selecting a new participant path.

This maintenance behavior depends on the complete member configuration. Add-Path alone does not replace the requirement to peer with all three route servers, and a retained path is useful only while its member next hop remains reachable on the peering fabric.

Receiving several paths does not by itself create ECMP. The member router must:

  1. retain the received Add-Path entries;
  2. consider the paths equivalent under its multipath rules;
  3. resolve distinct, reachable next hops on the peering LAN; and
  4. install more than one next hop in its forwarding table.

Attributes such as local preference, AS-path length, origin, MED, and IGP cost can prevent paths from being equal. Some platforms also require an option that permits multipath across different neighboring AS paths.

ECMP normally hashes flows across installed next hops. It does not make traffic symmetric: the remote network independently selects its return path.

Follow Enabling Add-Path and ECMP for the activation and verification sequence.