排除eBGP邻接建立故障

问题

防火墙和对等设备之间的外部边界网关协议(eBGP)邻接失败。观察到以下症状：

1.防火墙上的对等体状态为空闲：

fw# show bgp summary
BGP router identifier 192.0.2.2, local AS number 65001
BGP table version is 1, main routing table version 1
 
Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
198.51.100.2    4        65002 0       0              1    0    0 never  Idle

2.在接口捕获中只能看到来自对等设备的TCP SYN数据包：

fw# cap capo interface WAN-Telekom
fw# show cap capo
 
26 packets captured
 
   1: 06:22:44.990595       198.51.100.2.31242 > 192.0.2.2.179: S 2838607371:2838607371(0) win 16384 <mss 1460>
   2: 06:22:46.990152       198.51.100.2.31242 > 192.0.2.2.179: S 2838607371:2838607371(0) win 16384 <mss 1460>
   3: 06:22:50.991007       198.51.100.2.31242 > 192.0.2.2.179: S 2838607371:2838607371(0) win 16384 <mss 1460>
   4: 06:22:58.991281       198.51.100.2.31242 > 192.0.2.2.179: S 2838607371:2838607371(0) win 16384 <mss 1460>

3.成功建立到对等设备的IP地址的ICMP连接：

fw# ping 198.51.100.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 198.51.100.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/10 ms

这可以确认防火墙和对等设备之间的IP网络级别可达性。

4.调试级别的系统日志消息表示从对等设备丢弃的TCP请求：

fw# show logging
…
May 20 2026 06:32:58: %FTD-7-710005: TCP request discarded from 198.51.100.2/20217 to WAN-Telekom:192.0.2.2/179
May 20 2026 06:33:00: %FTD-7-710005: TCP request discarded from 198.51.100.2/20217 to WAN-Telekom:192.0.2.2/179
May 20 2026 06:33:04: %FTD-7-710005: TCP request discarded from 198.51.100.2/20217 to WAN-Telekom:192.0.2.2/179
May 20 2026 06:33:12: %FTD-7-710005: TCP request discarded from 198.51.100.2/20217 to WAN-Telekom:192.0.2.2/179

5. BGP调试显示“no route to peer”消息：

fw# debug ip bgp
BGP debugging is on
    for address family: IPv4 Unicast
Successfully set for module BGP at level 1
 
BGP: 198.51.100.2 Active open failed - no route to peer, open active delayed 21504ms (35000ms max, 60% jitter)

环境

拓扑

• 运行FTD 7.4.4并由安全防火墙管理中心(FMC)管理的Firepower 2110。 其他硬件平台和软件版本也可能受到影响。

• 防火墙通过连接到Internet服务提供商(ISP)的WAN-Telekom接口具有到对等地址的静态路由：

fw# show route 198.51.100.2

Routing entry for 198.51.100.2 255.255.255.255
  Known via "static", distance 1, metric 0
  Routing Descriptor Blocks:
  * 192.0.2.1, via WAN-Telekom
Route metric is 0, traffic share count is 1

• 防火墙具有BGP配置。对等体198.51.100.2具有不同的自治系统编号，因此是外部的：

fw# show run router
router bgp 65001
 bgp log-neighbor-changes
 bgp graceful-restart
 address-family ipv4 unicast
  neighbor 198.51.100.2 remote-as 65002
  neighbor 198.51.100.2 transport path-mtu-discovery disable
  neighbor 198.51.100.2 update-source WAN-Telekom
  neighbor 198.51.100.2 activate

分辨率

启用BGP邻居配置的Advanced部分中的Allow connections with neighbor that is not directly connected选项并将TTL Hops设置为255后，邻接关系即建立：

原因

默认情况下，防火墙允许直连对等体（即同一子网中的对等体）之间的eBGP邻接。要允许非直连对等体之间的邻接，必须启用允许与未直连的邻居的连接选项。此外，用户可以限制对等体的TTL跳数，并在从对等体收到的TCP数据包的IP报头中设置最小预期生存时间值。默认值为 1。

确认

1.未配置Allow connections with neighbor that is not directly connected选项：

fw# show bgp neighbors 198.51.100.2 | i External
  External BGP neighbor not directly connected.

2.配置了Allow connections with neighbor that is not directly connected选项，并且TTL Hops设置为1:

fw# show run router bgp | i 198.51.100.2
  neighbor 198.51.100.2 remote-as 65002
  neighbor 198.51.100.2 ebgp-multihop 1
  neighbor 198.51.100.2 transport path-mtu-discovery disable
  neighbor 198.51.100.2 update-source WAN-Telekom
  neighbor 198.51.100.2 activate


fw# show bgp neighbors 198.51.100.2 | i External
  External BGP neighbor not directly connected.

3.配置了Allow connections with neighbor that is not directly connected选项，并且TTL Hops设置为255:

fw# show run router bgp | i 198.51.100.2
  neighbor 198.51.100.2 remote-as 65002
  neighbor 198.51.100.2 ebgp-multihop 255
  neighbor 198.51.100.2 transport path-mtu-discovery disable
  neighbor 198.51.100.2 update-source WAN-Telekom
  neighbor 198.51.100.2 activate


fw# show bgp neighbors 198.51.100.2 | i External
  External BGP neighbor may be up to 255 hops away.

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修订历史记录

版本	发布日期	备注
1.0	20-May-2026	初始版本