Summary

  • ADD-PATH lets one BGP speaker advertise several paths for the same prefix without each new update replacing the previous one.
  • A Path Identifier is assigned locally and carries no inherent meaning; two identifiers are not proof of two independent network paths.
  • Capability negotiation is directional and scoped to an AFI/SAFI, while path selection, forwarding installation and physical diversity remain separate decisions.
  • A resilience claim needs a prefix-specific receipt that joins advertisements to egress, upstream, facility, shared-risk, FIB and measured-failover evidence.

Imagine a route-reflector dashboard showing two entries for the same customer prefix. The Path Identifiers differ, the ADD-PATH capability is green, and the change ticket records “redundant paths available.” A transport failure then removes both entries at once. The routes came from separate advertisements but converged on the same edge, the same upstream handoff and the same metro circuit.

This is a hypothetical operating trace, not a report about a named network. Its lesson is narrower: multiplicity in a routing table is not a topology audit.

What ADD-PATH actually preserves

RFC 7911 extends BGP so a speaker can advertise multiple paths for the same address prefix or NLRI without every new advertisement implicitly replacing the last one. It prepends a four-octet Path Identifier to the NLRI. The relevant key becomes prefix plus Path Identifier.

That identifier is local to the assigning speaker. A speaker that re-advertises a route generates its own value, and a receiver should not assume the number has any particular semantics. It is therefore unsafe to turn 17 and 29 into claims such as “two carriers,” “two sites” or “two conduits.” The identifiers distinguish advertisements on a session; they do not describe the physical dependencies below them.

The capability is also directional and scoped. For an AFI/SAFI, a speaker announces whether it can receive multiple paths, send them, or do both. Multiple-path encoding is used only when the sender's and receiver's advertised roles align. A generic “ADD-PATH enabled” inventory line can hide a family, direction or session where the feature is not active.

RFC 7911 says additional paths can provide potential alternates or backups and can improve convergence or help with oscillation. “Potential” matters. The standard does not define the selection algorithm that decides which paths to advertise, does not promise that a receiver installs all of them in its FIB, and does not prove that traffic can move between them.

Where apparent diversity collapses

RFC 6774 frames the distribution of more than one BGP path around convergence, reachability restoration and load balancing. It also treats diversity as something that has to be selected and propagated, including diversity among peers, rather than something created by the mere presence of two rows.

Two advertisements can differ in AS_PATH yet share the same border router, line card, optical system or building. They can arrive from two peer addresses belonging to one upstream and cross the same leased circuit. A route reflector can expose two candidates whose next hops recursively resolve through one IGP path. Even genuinely different external routes may collapse onto one forwarding adjacency after policy and best-path processing.

Partial deployment adds another boundary. A capable session may preserve multiple routes at one hop while another reflector or edge exports only one. Path Identifiers can change across a control-plane restart. A packet trace taken without the capability exchange can even be decoded incorrectly. The evidence must therefore name the session, AFI/SAFI, direction and observation point.

A RIB row is not a forwarding result

The resilience claim becomes stronger only after the control-plane candidates are connected to forwarding state. For each prefix, record which routes were received, which were eligible, which became best or backup, which next hops resolved, and which entries reached the RIB and FIB. Then identify the egress router, upstream ASN, facility, circuit and shared-risk group beneath each candidate.

Finally, observe traffic. A controlled withdrawal or failure exercise should show detection time, withdrawal or reselection, FIB transition, loss interval and the path used after recovery. Without that record, ADD-PATH proves that a protocol can carry extra choices—not that the network has an independent escape route.