Guide

ADD-PATH and PATHS-LIMIT

ADD-PATH (RFC 7911) allows multiple paths per prefix by including a Path Identifier with each NLRI. This is useful for route servers that need to forward all available paths, not just the best one.

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 behavior

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.

Interaction with Route Reflection

ADD-PATH is particularly useful with 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, all received paths are forwarded, allowing downstream routers to make their own best-path decisions.