在ECS CentOS上配置L2tP服务，一直无法连接，提示“no connection has been authorized with policy=PSK”

ECS上配置L2TP服务后，执行ipsec verify，正常：
Checking your system to see if IPsec got installed and started correctly:
Version check and ipsec on-path [OK]
Linux Openswan U2.6.38/K3.10.0-514.26.2.el7.x86_64 (netkey)
Checking for IPsec support in kernel [OK]
SAref kernel support [N/A]
NETKEY: Testing XFRM related proc values [OK]

[OK]
[OK]

Hardware RNG detected, testing if used properly [OK]
Checking that pluto is running [OK]
Pluto listening for IKE on udp 500 [OK]
Pluto listening for NAT-T on udp 4500 [OK]
Checking for 'ip' command [OK]
Checking /bin/sh is not /bin/dash [OK]
Checking for 'iptables' command [OK]
Opportunistic Encryption Support [DISABLED]

但是连接L2TP时，/var/log/secure中出现下列错误：
Nov 17 11:43:15 izj6c6h25ghj5n6vo0emwnz pluto[10722]: packet from 124.205.127.38:500: ignoring Vendor ID payload [IKE CGA version 1]
Nov 17 11:43:15 izj6c6h25ghj5n6vo0emwnz pluto[10722]: packet from 124.205.127.38:500: initial Main Mode message received on 172.31.232.230:500 but no connection has been authorized with policy=PSK
Nov 17 11:43:16 izj6c6h25ghj5n6vo0emwnz pluto[10722]: packet from 124.205.127.38:500: phase 1 message is part of an unknown exchange
折腾了3天仍然不知原因，能否指点一下？
非常感谢！