additional-path

2025/12/2动态路由7.X.X大约 13 分钟

additional-path

功能简介

在 Hub-Spoke 场景中，每个 Spoke 往往通过多个链路（如 ISP1、ISP2、MPLS）与 Hub 建立多条 VPN 隧道。当 BGP 建立后，Hub 在向 Spoke 反射其他 Spoke 的路由时，可能只向 Spoke 通告单一路径，使得 Spoke 无法基于多路径进行更优的路径选择或负载分担。

通过启用 BGP Additional Path 机制：additional-path-select 与 adv-additional-path，Hub 可以向 Spoke 通告多个等价路径，使 Spoke 能够正确学习多条路由，提升 Hub-Spoke 的性能与稳定性。

网络拓扑

IPSec VPN：
- Hub 有 3 条 ISP，每条上线 1 条 dynamic 模式 IPSec，供 Spoke 拨入。
- Spoke1、Spoke2 各有 3 条对应 ISP，与 Hub 的 3 条 IPSec 隧道一一对应连接。
BGP：
- Spoke 通过 3 条 IPSec 隧道分别与 Hub 的 3 条隧道建立 iBGP 邻居，并发布各自内网路由。
- Hub 作为 BGP RR，在 Spoke 之间反射 iBGP 路由。
- Hub 需要在 3 个 IPSec 隧道上负载去往 Spoke 的路由，Spoke 需要在 3 个 IPSec 隧道上负载去往 Hub 与其他 Spoke 的路由。

配置步骤

基础网络与安全策略配置（略）。

Hub

配置 Hub 的 3 条 IPSec VPN dynamic 模式隧道，用于 Spoke 拨入。开启exchange-interface-ip用于交换 Tunnel 接口 IP，关闭add-route功能。

config vpn ipsec phase1-interface
    edit "vpn_line1"
        set type dynamic
        set interface "port2"
        set ike-version 2
        set peertype any
        set net-device disable
        set exchange-interface-ip enable
        set proposal aes128-sha256
        set add-route disable
        set dpd on-idle
        set psksecret xxxxxx
        set dpd-retryinterval 60
    next
    edit "vpn_line2"
        set type dynamic
        set interface "port3"
        set ike-version 2
        set peertype any
        set net-device disable
        set exchange-interface-ip enable
        set proposal aes128-sha256
        set add-route disable
        set dpd on-idle
        set psksecret xxxxxx
        set dpd-retryinterval 60
    next
    edit "vpn_line3"
        set type dynamic
        set interface "port4"
        set ike-version 2
        set peertype any
        set net-device disable
        set exchange-interface-ip enable
        set proposal aes128-sha256
        set add-route disable
        set dpd on-idle
        set psksecret xxxxxx
        set dpd-retryinterval 60
    next
end

config vpn ipsec phase2-interface
    edit "vpn_line1"
        set phase1name "vpn_line1"
        set proposal aes128-sha1
        set keepalive enable
    next
    edit "vpn_line2"
        set phase1name "vpn_line2"
        set proposal aes128-sha1
        set keepalive enable
    next
    edit "vpn_line3"
        set phase1name "vpn_line3"
        set proposal aes128-sha1
        set keepalive enable
    next
end

根据拓扑中的信息配置 Hub 的 IPSec 隧道 IP 地址，用于与 Spoke 建立 IBGP 邻居。

config system interface
    edit "vpn_line1"
        set ip 169.254.10.254 255.255.255.255
        set allowaccess ping
        set remote-ip 169.254.10.254 255.255.255.0
    next
    edit "vpn_line2"
        set ip 169.254.20.254 255.255.255.255
        set allowaccess ping
        set remote-ip 169.254.20.254 255.255.255.0
    next
    edit "vpn_line3"
        set ip 169.254.30.254 255.255.255.255
        set allowaccess ping
        set remote-ip 169.254.30.254 255.255.255.0
    next
end

在 Hub 的 3 个 IPSec 隧道上分别配置 BGP 邻居组和邻居范围，用于接受 Spoke 的 iBGP 邻居建立，在每个邻居组中开启 RR 功能，并发布自身内网路由。

提示

由于需要多线路负载 iBGP 路由，需要开启ibgp-multipath，原理请参考路由 → 动态路由 → BGP → BGP 多路径章节。

config router bgp
    set as 10086
    set router-id 169.254.10.254
    set ibgp-multipath enable
    config neighbor-group
        edit "vpn_line1"
            set soft-reconfiguration enable
            set interface "vpn_line1"
            set remote-as 10086
            set route-reflector-client enable
        next
        edit "vpn_line2"
            set soft-reconfiguration enable
            set interface "vpn_line2"
            set remote-as 10086
            set route-reflector-client enable
        next
        edit "vpn_line3"
            set soft-reconfiguration enable
            set interface "vpn_line3"
            set remote-as 10086
            set route-reflector-client enable
        next
    end
    config neighbor-range
        edit 1
            set prefix 169.254.10.0 255.255.255.0
            set neighbor-group "vpn_line1"
        next
        edit 2
            set prefix 169.254.20.0 255.255.255.0
            set neighbor-group "vpn_line2"
        next
        edit 3
            set prefix 169.254.30.0 255.255.255.0
            set neighbor-group "vpn_line3"
        next
    end
    config network
        edit 1
            set prefix 10.10.254.0 255.255.255.0
        next
    end
end

Spoke1

配置 Spoke1 的 3 条 IPSec VPN 隧道，分别对接 Hub 的 3 条 IPSec 隧道。开启exchange-interface-ip用于交换 Tunnel 接口 IP。

config vpn ipsec phase1-interface
    edit "vpn_line1"
        set interface "port2"
        set ike-version 2
        set peertype any
        set exchange-interface-ip enable
        set proposal aes128-sha256
        set dpd on-idle
        set remote-gw 202.103.7.2
        set psksecret xxxxxx
    next
    edit "vpn_line2"
        set interface "port3"
        set ike-version 2
        set peertype any
        set exchange-interface-ip enable
        set proposal aes128-sha256
        set dpd on-idle
        set remote-gw 202.103.8.2
        set psksecret xxxxxx
    next
    edit "vpn_line3"
        set interface "port4"
        set ike-version 2
        set peertype any
        set exchange-interface-ip enable
        set proposal aes128-sha256
        set dpd on-idle
        set remote-gw 202.103.9.2
        set psksecret xxxxxx
    next
end

config vpn ipsec phase2-interface
    edit "vpn_line1"
        set phase1name "vpn_line1"
        set proposal aes128-sha1
        set auto-negotiate enable
    next
    edit "vpn_line2"
        set phase1name "vpn_line2"
        set proposal aes128-sha1
        set auto-negotiate enable
    next
    edit "vpn_line3"
        set phase1name "vpn_line3"
        set proposal aes128-sha1
        set auto-negotiate enable
    next
end

根据拓扑中的信息配置 Spoke1 的 IPSec 隧道 IP 地址，用于与 Hub 建立 iBGP 邻居。

config system interface
    edit "vpn_line1"
        set ip 169.254.10.1 255.255.255.255
        set allowaccess ping
        set remote-ip 169.254.10.254 255.255.255.0
    next
    edit "vpn_line2"
        set ip 169.254.20.1 255.255.255.255
        set allowaccess ping
        set remote-ip 169.254.20.254 255.255.255.0
    next 
    edit "vpn_line3"
        set ip 169.254.30.1 255.255.255.255
        set allowaccess ping
        set remote-ip 169.254.30.254 255.255.255.0
    next
end

在 Spoke1 的 3 个 IPSec 隧道上分别配置 BGP 邻居，用于向 Hub 发起 iBGP 邻居建立，发布自身内网路由。

提示

由于需要多线路负载 iBGP 路由，需要开启ibgp-multipath，原理请参考路由 → 动态路由 → BGP → BGP 多路径章节。

config router bgp
    set as 10086
    set router-id 169.254.10.1
    set ibgp-multipath enable
    config neighbor
        edit "169.254.10.254"
            set soft-reconfiguration enable
            set interface "vpn_line1"
            set remote-as 10086
            set update-source "vpn_line1"
        next
        edit "169.254.20.254"
            set soft-reconfiguration enable
            set interface "vpn_line2"
            set remote-as 10086
            set update-source "vpn_line2"
        next
        edit "169.254.30.254"
            set soft-reconfiguration enable
            set interface "vpn_line3"
            set remote-as 10086
            set update-source "vpn_line3"
        next
    end
    config network
        edit 1
            set prefix 10.10.1.0 255.255.255.0
        next
    end
end

Spoke2

与 Spoke1 基本一致，点击展开查看。

配置 Spoke2 的 3 条 IPSec VPN 隧道，分别对接 Hub 的 3 条 IPSec 隧道。开启exchange-interface-ip用于交换 Tunnel 接口 IP。

config vpn ipsec phase1-interface
    edit "vpn_line1"
        set interface "port2"
        set ike-version 2
        set peertype any
        set exchange-interface-ip enable
        set proposal aes128-sha256
        set dpd on-idle
        set remote-gw 202.103.7.2
        set psksecret xxxxxx
    next
    edit "vpn_line2"
        set interface "port3"
        set ike-version 2
        set peertype any
        set exchange-interface-ip enable
        set proposal aes128-sha256
        set dpd on-idle
        set remote-gw 202.103.8.2
        set psksecret xxxxxx
    next
    edit "vpn_line3"
        set interface "port4"
        set ike-version 2
        set peertype any
        set exchange-interface-ip enable
        set proposal aes128-sha256
        set dpd on-idle
        set remote-gw 202.103.9.2
        set psksecret xxxxxx
    next
end

config vpn ipsec phase2-interface
    edit "vpn_line1"
        set phase1name "vpn_line1"
        set proposal aes128-sha1
        set auto-negotiate enable
    next
    edit "vpn_line2"
        set phase1name "vpn_line2"
        set proposal aes128-sha1
        set auto-negotiate enable
    next
    edit "vpn_line3"
        set phase1name "vpn_line3"
        set proposal aes128-sha1
        set auto-negotiate enable
    next
end

根据拓扑中的信息配置 Spoke2 的 IPSec 隧道 IP 地址，用于与 Hub 建立 iBGP 邻居。

config system interface
    edit "vpn_line1"
        set ip 169.254.10.2 255.255.255.255
        set allowaccess ping
        set remote-ip 169.254.10.254 255.255.255.0
    next
    edit "vpn_line2"
        set ip 169.254.20.2 255.255.255.255
        set allowaccess ping
        set remote-ip 169.254.20.254 255.255.255.0
    next 
    edit "vpn_line3"
        set ip 169.254.30.2 255.255.255.255
        set allowaccess ping
        set remote-ip 169.254.30.254 255.255.255.0
    next
end

在 Spoke2 的 3 个 IPSec 隧道上分别配置 BGP 邻居，用于向 Hub 发起 iBGP 邻居建立，发布自身内网路由。

提示

由于需要多线路负载 iBGP 路由，需要开启ibgp-multipath，原理请参考路由 → 动态路由 → BGP → BGP 多路径章节。

config router bgp
    set as 10086
    set router-id 169.254.10.2
    set ibgp-multipath enable
    config neighbor
        edit "169.254.10.254"
            set soft-reconfiguration enable
            set interface "vpn_line1"
            set remote-as 10086
            set update-source "vpn_line1"
        next
        edit "169.254.20.254"
            set soft-reconfiguration enable
            set interface "vpn_line2"
            set remote-as 10086
            set update-source "vpn_line2"
        next
        edit "169.254.30.254"
            set soft-reconfiguration enable
            set interface "vpn_line3"
            set remote-as 10086
            set update-source "vpn_line3"
        next
    end
    config network
        edit 1
            set prefix 10.10.2.0 255.255.255.0
        next
    end
end

初始状态

在 Hub 查看路由表中的 BGP 路由，可以看到从 Spoke1 和 Spoke2 的 3 条路径分别学到了 Spoke1 和 Spoke2 发布的 BGP 路由，符合预期。

Hub # get router info routing-table all
Routing table for VRF=0
B       10.10.1.0/24 [200/0] via 169.254.10.1 (recursive is directly connected, vpn_line1), 10:31:39, [1/0]
                     [200/0] via 169.254.20.1 (recursive is directly connected, vpn_line2), 10:31:39, [1/0]
                     [200/0] via 169.254.30.1 (recursive is directly connected, vpn_line3), 10:31:39, [1/0]
B       10.10.2.0/24 [200/0] via 169.254.10.2 (recursive is directly connected, vpn_line1), 10:31:19, [1/0]
                     [200/0] via 169.254.20.2 (recursive is directly connected, vpn_line2), 10:31:19, [1/0]
                     [200/0] via 169.254.30.2 (recursive is directly connected, vpn_line3), 10:31:19, [1/0]
......
S       169.254.10.0/24 [5/0] via vpn_line1 tunnel 202.103.7.2, [1/0]
C       169.254.10.1/32 is directly connected, vpn_line1
......

在 Spoke1 的路由表中查看 BGP 路由，从 Hub 学到的 3 条 Hub 内网路由可以在 3 条 IPSec 隧道负载分担，符合预期。但经过 Hub 反射的 Spoke2 的 3 条 BGP 路由（10.10.2.0/24，[3]），只有一个下一跳（169.254.10.2，Spoke2 的 vpn_line1）。（Spoke2 也是一样的情况）

Spoke1 # get router info routing-table bgp
Routing table for VRF=0
B       10.10.254.0/24 [200/0] via 169.254.10.254 (recursive via vpn_line1 tunnel 202.103.7.2), 00:00:17, [1/0]
                       [200/0] via 169.254.20.254 (recursive via vpn_line2 tunnel 202.103.8.2), 00:00:17, [1/0]
                       [200/0] via 169.254.30.254 (recursive via vpn_line3 tunnel 202.103.9.2), 00:00:17, [1/0]
B       10.10.2.0/24 [200/0] via 169.254.10.2 [3] (recursive via vpn_line1 tunnel 202.103.7.2), 00:00:43, [1/0]

从 Spoke1 的 BGP 路由状态中可以看到，虽然 Spoke1 确实从 3 个隧道分别收到了 Hub 反射的 Spoke2 路由，但下一跳均是 Spoke2 vpn_line1 的 IP，导致路由在递归时全部递归到 vpn_line1。

Spoke1 # get router info bgp network 10.10.2.0
VRF 0 BGP routing table entry for 10.10.2.0/24
Paths: (3 available, best #1, table Default-IP-Routing-Table)
  Not advertised to any peer
  Original VRF 0
  Local
    169.254.10.2 from 169.254.10.254 (169.254.10.2)
      Origin IGP distance 200 metric 0, localpref 100, valid, internal, best
      Originator: 169.254.10.2, Cluster list: 169.254.10.254 
      Last update: Wed Dec  3 16:23:58 2025

  Original VRF 0
  Local
    169.254.10.2 from 169.254.30.254 (169.254.10.2)
      Origin IGP distance 200 metric 0, localpref 100, valid, internal
      Originator: 169.254.10.2, Cluster list: 169.254.10.254 
      Last update: Wed Dec  3 16:23:57 2025

  Original VRF 0
  Local
    169.254.10.2 from 169.254.20.254 (169.254.10.2)
      Origin IGP distance 200 metric 0, localpref 100, valid, internal
      Originator: 169.254.10.2, Cluster list: 169.254.10.254 
      Last update: Wed Dec  3 16:23:57 2025

在 Hub 查看 3 条隧道上向 Spoke1 的 3 个邻居反射的 Spoke2 的 BGP 路由（10.10.2.0/24），下一跳均为 169.254.10.2（Spoke2 的 vpn_line1）。

Hub # get router info bgp neighbors 169.254.10.1 advertised-routes | grep 10.10.2.0/24
*>i10.10.2.0/24     169.254.10.2                  100      0        0 i <-/->

Hub # get router info bgp neighbors 169.254.20.1 advertised-routes | grep 10.10.2.0/24
*>i10.10.2.0/24     169.254.10.2                  100      0        0 i <-/->

Hub # get router info bgp neighbors 169.254.30.1 advertised-routes | grep 10.10.2.0/24
*>i10.10.2.0/24     169.254.10.2                  100      0        0 i <-/->

这是由于当前配置下，虽然 Hub 的路由表里可看到 3 条来自 Spoke2 的 BGP 路由，但 Hub 仅向 Spoke1 通告最佳路径（Best Path，169.254.10.2），导致 Spoke1 学习到 3 条 Spoke2 的同一个下一跳路由。

Hub # get router info bgp network 10.10.2.0
VRF 0 BGP routing table entry for 10.10.2.0/24
Paths: (3 available, best #3, table Default-IP-Routing-Table)
  Advertised to peer-groups:
  Hub_line1 Hub_line2 Hub_line3
  Original VRF 0
  Local, (Received from a RR-client)
    169.254.30.2 from 169.254.30.2 (169.254.10.2)
      Origin IGP distance 200 metric 0, localpref 100, valid, internal
      Last update: Tue Dec  2 14:56:56 2025

  Original VRF 0
  Local, (Received from a RR-client)
    169.254.20.2 from 169.254.20.2 (169.254.10.2)
      Origin IGP distance 200 metric 0, localpref 100, valid, internal
      Last update: Tue Dec  2 14:56:40 2025

  Original VRF 0
  Local, (Received from a RR-client)
    169.254.10.2 from 169.254.10.2 (169.254.10.2)
      Origin IGP distance 200 metric 0, localpref 100, valid, internal, best
      Last update: Tue Dec  2 14:56:35 2025

开启 additional-path

启用 BGP Additional Path 机制，使 Hub 和 Spoke 均能通告与学习多个 BGP 等价路径。

在 Hub 上配置 BGP additional-path。
```
config router bgp
    set additional-path enable
    set additional-path-select 3
    config neighbor-group
        edit "Hub_line1"
            set additional-path send
            set adv-additional-path 3
        next
        edit "Hub_line2"
            set additional-path send
            set adv-additional-path 3
        next
        edit "Hub_line3"
            set additional-path send
            set adv-additional-path 3
        next
    end
end
```
- set additional-path enable：BGP additional-path 路由学习的总开关，在 Hub（RR）上需要开启。
- set additional-path-select 3：默认值为 2。相同 BGP 属性但下一跳不一样的路由，最多选取（select）3 条作为best路由。
- set additional-path send：在 BGP 协商时，向 BGP 邻居通告在发送方向启用IPv4 additional-path capability，也就是自身具有发送 additional-path 路由的能力，也可以使用both。
- set adv-additional-path 3：默认值为 2。向每个邻居通告的 IPv4 路由最大路径数量。仅在set additional-path send或set additional-path both时可以配置。
在 Spoke1 和 Spoke2 上配置 BGP additional-path，仅需要在邻居配置中配置set additional-path receive。
```
config router bgp
    config neighbor
        edit "169.254.10.254"
            set additional-path receive
        next
        edit "169.254.20.254"
            set additional-path receive
        next
        edit "169.254.30.254"
            set additional-path receive
        next
    end
end
```
- set additional-path send：在 BGP 协商时，向 BGP 邻居通告在接收方向启用IPv4 additional-path capability，也就是自身具有接收 additional-path 路由的能力，也可以使用both。
  重要
  additional-path send/receive/both为 BGP 邻居协商时交换的 BGP capability，需要 BGP 邻居两端同时开启才能生效，可以根据实际需求进行模式选择：
  - 一端配置为send，另一端配置为receive/both。
  - 一端配置为both，另一端配置为send/receive。
  - 两端均配置为both。

等待 Hub 的 BGP 路由重新收敛，查看 Hub 从 3 条 IPSec 隧道上学到的 Spoke2 的 3 条 BGP 路由，均变为具有best标记的路由（Hub 上配置的set additional-path-select 3作用效果）。

Hub # get router info bgp network 10.10.2.0
VRF 0 BGP routing table entry for 10.10.2.0/24
Paths: (3 available, best #3, table Default-IP-Routing-Table)
  Advertised to peer-groups:
  Hub_line1 Hub_line2 Hub_line3
  Original VRF 0
  Local, (Received from a RR-client)
    169.254.30.2 from 169.254.30.2 (169.254.10.2)
      Origin IGP distance 200 metric 0, localpref 100, valid, internal, best
      Advertised Path ID: 3
       Last update: Tue Dec  2 16:21:36 2025

  Original VRF 0
  Local, (Received from a RR-client)
    169.254.20.2 from 169.254.20.2 (169.254.10.2)
      Origin IGP distance 200 metric 0, localpref 100, valid, internal, best
      Advertised Path ID: 2
       Last update: Tue Dec  2 16:21:26 2025

  Original VRF 0
  Local, (Received from a RR-client)
    169.254.10.2 from 169.254.10.2 (169.254.10.2)
      Origin IGP distance 200 metric 0, localpref 100, valid, internal, best
      Advertised Path ID: 1
       Last update: Tue Dec  2 16:21:09 2025

在 Hub 查看 3 条隧道上向 Spoke1 的 3 个邻居反射的 Spoke2 的 BGP 路由（10.10.2.0/24），可以看到 Hub 在每个邻居上都反射了 3 个路径的路由（Hub 上配置的set adv-additional-path 3作用效果）。

Hub # get router info bgp neighbors 169.254.10.1 advertised-routes | grep 10.10.2.0
*>i10.10.2.0/24     169.254.30.2                  100      0        0 i <-/3>
*>i10.10.2.0/24     169.254.20.2                  100      0        0 i <-/2>
*>i10.10.2.0/24     169.254.10.2                  100      0        0 i <-/1>

Hub # get router info bgp neighbors 169.254.20.1 advertised-routes | grep 10.10.2.0
*>i10.10.2.0/24     169.254.30.2                  100      0        0 i <-/3>
*>i10.10.2.0/24     169.254.20.2                  100      0        0 i <-/2>
*>i10.10.2.0/24     169.254.10.2                  100      0        0 i <-/1>

Hub # get router info bgp neighbors 169.254.30.1 advertised-routes | grep 10.10.2.0
*>i10.10.2.0/24     169.254.30.2                  100      0        0 i <-/3>
*>i10.10.2.0/24     169.254.20.2                  100      0        0 i <-/2>
*>i10.10.2.0/24     169.254.10.2                  100      0        0 i <-/1>

在 Spoke1 查看路由表中的 BGP 路由，可以看到 Spoke1 在每个隧道上都学到了 3 条路径的 Spoke2 发布的 BGP 路由（Spoke 上配置的set additional-path receive作用效果）。

Spoke1 # get router info routing-table bgp
Routing table for VRF=0
B       10.10.2.0/24 [200/0] via 169.254.10.2 [3] (recursive via vpn_line1 tunnel 202.103.7.2), 00:17:34, [1/0]
                     [200/0] via 169.254.20.2 [3] (recursive via vpn_line2 tunnel 202.103.8.2), 00:17:34, [1/0]
                     [200/0] via 169.254.30.2 [3] (recursive via vpn_line3 tunnel 202.103.9.2), 00:17:34, [1/0]
......

在 Hub 上查看邻居 Spoke1 的状态，可以看到邻居 Spoke1 具有“Additional Path”的接收能力，而 Hub 自身具有“Additional Path”的发送能力。

Hub # get router info bgp neighbors 169.254.10.1 | grep Additional -A 2
    Additional Path:
      Send-mode: advertised
      Receive-mode: received

提示

如果 Hub 配置为set additional-path both，则会显示：

Hub # get router info bgp neighbors 169.254.10.1 | grep Additional -A 2
    Additional Path:
      Send-mode: advertised
      Receive-mode: advertised, received

在 Spoke1 上查看邻居 Spoke1 的状态，可以看到邻居 Hub 具有“Additional Path”的发送能力，而 Spoke1 自身具有“Additional Path”的接收能力。

Spoke1 # get router info bgp neighbors 169.254.10.254 | grep Additional -A 2
    Additional Path:
      Send-mode: received
      Receive-mode: advertised

注意事项

BGP 只有在这些关键属性完全一致时，才会把路径视为 ECMP 等价路径：
- Weight
- Local Preference
- AS Path
- Origin
- MED
- IGP Metric
配置 additional-path 后，如 Spoke 仍只学到一个路径，可启用 BGP 调试。若看到Duplicate route ignored，则说明多路径属性不一致，需要检查以上字段是否完全相同。
```
diagnose ip router bgp all enable
diagnose ip router bgp level info
diagnose debug enable
```

附件

Hub 与 Spoke1 BGP 邻居协商 additional-path capability 的报文，如下所示，Hub 向 Spoke1 宣告的IPv4 additional-path capability为send（bgp_additional_path.pcap）。

总结

config router bgp：
- additional-path enable：开启全局 additional-path 能力，使 RR 本地 BGP 路由表可以同时保留多条等价路径，通常在 RR（Hub）上开启。
- additional-path-select <n>：默认值为 2。依赖 additional-path enable，指定最多从多少条等价路径中“选取（select）”并标记为 best，用于控制本地路由表中可保留的等价路径数量，通常在 RR 上配置为与实际链路数一致。
config neighbor-group / neighbor：
- set additional-path send / receive / both：定义在 BGP capability 协商中，本端是否向对端通告具备发送或接收 additional-path 能力。只有双方能力匹配（send ↔ receive/both）后，多路径通告才会生效。典型用法是：RR 使用 send，RR Client 使用 receive。
- adv-additional-path <n>：默认值为 2。决定 RR 本端针对单个邻居最多反射多少条路径。通常与 additional-path-select 数量相同，使 RR 可以将多条等价路径同时反射给下游 Spoke。