今天测试一双路由平台(双线路平台)、（香港出口和联通出口）；发现通过联通平台vpn接入能ping通外网的IP，但是通过香港平台vpn接入ping外网的IP出现TTL expired in transit.错误，于是traceroute测试发现是路由环路导致TTL超时了。以下是测试时的出错信息：
C:>ping www.itkylin.com Pinging www.itkylin.com [216.34.181.92] with 32 bytes of data: Reply from 172.30.0.4: TTL expired in transit. Reply from 172.30.0.4: TTL expired in transit. Reply from 172.30.0.4: TTL expired in transit. Reply from 172.30.0.4: TTL expired in transit. Ping statistics for 216.34.181.92: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 0ms, Maximum = 0ms, Average = 0ms C:>tracert -d www.itkylin.com Tracing route to www.itkylin.com [216.34.181.92] over a maximum of 30 hops 1 45 ms 43 ms 44 ms 192.168.0.1 2 145 ms 46 ms 51 ms 112.93.248.12 3 * * * Request timed out. 4 48 ms 43 ms 45 ms 172.30.0.4 5 * * * Request timed out. 6 48 ms 45 ms 47 ms 172.30.0.4 7 * * * Request timed out. 8 50 ms 49 ms 45 ms 172.30.0.4 9 * * * Request timed out. 10 56 ms 49 ms 47 ms 172.30.0.4 11 * * * Request timed out. 12 119 ms 54 ms 51 ms 172.30.0.4 13 * * * Request timed out. 14 251 ms 51 ms 49 ms 172.30.0.4 15 * * * Request timed out. 16 55 ms 55 ms 51 ms 172.30.0.4 17 ^C C:>
通常导致TTL expired in transit.的原因有两个：
1）TTL值太小，TTL值小于你和对方主机之间经过的路由器数目；
2）路由器数量太多，经过路由器的数量大于TTL值。
问题分析：
根据traceroute得出的路由路径信息，发现路由卡死在172.30.0.4这个路由上，在到这个路由时产生了环路，互相踢皮球，确定是双路由平台(双线路平台)提供商的问题，提供商香港路由平台的架构我们是不清楚的，所以没办法向其建议解决，只能将发生的情况通知他们让他们做进一骤的检测！