Guide

ADD-PATH and PATHS-LIMIT

ADD-PATH (RFC 7911) allows multiple paths per prefix by including a Path Identifier with each NLRI. Route servers use it to forward all available paths rather than one best path.

PATHS-LIMIT (draft-abraitis-idr-addpath-paths-limit) lets a receiver advertise the maximum number of paths it wants per prefix per family, preventing uncontrolled path proliferation.

Configuration

All ADD-PATH and PATHS-LIMIT config lives under session > capability > add-path.

Default Direction and Limit

Enable ADD-PATH for all negotiated families, with an optional default path count limit:

capability {
    add-path {
        direction send/receive;
        limit 10;
    }
}

The limit on the container is inherited by all families. Per-family entries can override it.

Per-Family Override

Override direction, set path count limits, or set negotiation mode per family:

capability {
    add-path {
        direction send;
        family {
            ipv4/unicast {
                direction send/receive;
                limit 10;
            }
            ipv6/unicast {
                direction receive;
                mode require;
            }
        }
    }
}

Full Example

bgp {
    peer transit-a {
        connection {
            remote { ip 10.0.0.1; }
            local { ip 10.0.0.2; }
        }
        session {
            asn { local 65000; remote 65001; }
            capability {
                add-path {
                    direction send/receive;
                    family {
                        ipv4/unicast { limit 10; }
                    }
                }
            }
            family {
                ipv4/unicast
                ipv6/unicast
            }
        }
    }
}

Direction and Mode

Direction	Meaning
`send`	Advertise multiple paths to peer
`receive`	Accept multiple paths from peer
`send/receive`	Both directions

Mode	Meaning
`enable` (default)	Negotiate ADD-PATH if peer supports it
`require`	Reject peer if it does not support ADD-PATH
`refuse`	Reject peer if it advertises ADD-PATH
`disable`	Do not negotiate ADD-PATH for this family

PATHS-LIMIT

The limit leaf on a per-family entry advertises a PATHS-LIMIT capability (code 76) for that family. The value (1-65535) is the maximum number of paths per prefix the receiver wants.

Setting	Effect
✕	✕
`limit 10`	Peer will send at most 10 paths per prefix
`limit 1`	Effectively single-path behaviour

How It Works

When ADD-PATH is negotiated, each NLRI is prefixed with a 4-byte Path Identifier. This allows the same prefix (e.g., 10.0.0.0/24) to appear multiple times with different path IDs, each carrying different attributes.

Wire Format

Without ADD-PATH:

[prefix-length][prefix-bytes]

With ADD-PATH:

[4-byte path-id][prefix-length][prefix-bytes]

Encoding Context

Peers that negotiate the same ADD-PATH modes share an encoding context (ContextID). The route server can forward wire bytes unchanged between peers with matching contexts, avoiding re-encoding.

Route Withdrawal

To withdraw a specific path, the withdrawal NLRI includes the same path ID used in the announcement. Withdrawing without a path ID removes all paths for that prefix.

Ze Generates Its Own Path Identifiers

RFC 7911 Section 2 requires a speaker that re-advertises a route to generate its own Path Identifier, and to assign it so that (prefix, identifier) uniquely names a path advertised to a neighbor. Both forward rails do that instead of relaying the identifier the source chose.

Each route-server client picks its identifiers alone. Two clients that both pick 1 for one prefix would reach a third client as one (prefix, identifier) pair, and RFC 7911 Section 5 makes the receiver treat the second as a replacement for the first, so one path is lost. A source that negotiated no ADD-PATH sends every path under identifier 0, so without regeneration every path from every ordinary client reached an ADD-PATH client as (prefix, 0).

Property	Behavior
Key	The path at ingress: the source that sent it and the identifier that source used. Never the message, never the attributes
Withdrawal	A withdrawn route carries no path attributes, so the ingress key is the only key that can be recomputed when the path leaves. The withdrawal names the identifier Ze advertised
Re-announcement	A source that re-announces one path with changed attributes keeps the identifier it already has, so the destination sees a replacement rather than a duplicate
Release	Identifiers are released at peer removal, not at session down. A reconnecting peer re-announces under the identifiers its destinations already hold
Zero	Minted and accepted like any other value, which RFC 7911 Section 3 requires

Regeneration runs whenever either side of the forward frames identifiers. A session where neither side negotiated ADD-PATH keeps its zero-copy forward.

Interaction with Route Reflection

ADD-PATH fits the route server plugin (bgp-rs). Without ADD-PATH, the route server can only forward one path per prefix to each peer. With ADD-PATH, it forwards all received paths, and downstream routers make their own best-path decisions.