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BGP timer协商实验_bgp的keepalive周期
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1.实验目的
默认情况下，BGP的keepalive报文周期为60秒，保持周期（holdtimer）为180秒。出于加速或减缓收敛的目的，可以通过bgp timers keepalive holdtime全局命令或者针对具体邻居的neighborxxxx timer keepalive holdtime命令进行调整。
但与OSPF等IGP协议不同，BGP邻居之间的timer可以不一致，通过TCP连接建立成功后的open报文交互中，协商使用最低值。
2.实验结果
（1）R1采用默认设置，keepalive60秒，hold time 180秒。R2进行调整，keepalive 10秒，hold time 30秒。
配置完成后，并不生效。因为相关参数的传递由open报文负责，而在BGP邻居状态使能的情况下，只有keepalive和update报文交互。使用clear ip bgp * 命令后，两端协商timer完成，如表5所示。
表4 BGP邻居timer情况（一）
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R2#sh ip bgp neighbors 1.1.1.1 BGP neighbor is 1.1.1.1, remote AS 1, internal link BGP version 4, remote router ID 1.1.1.1 BGP state = Established, up for 00:00:19 Last read 00:00:09, last write 00:00:01, hold time is 30, keepalive interval is 10 seconds Configured hold time is 30, keepalive interval is 10 seconds !!!!在R1上查看R2的邻居状态情况 R1# sh ip bgp neighbors 2.2.2.2 BGP neighbor is 2.2.2.2, remote AS 1, internal link BGP version 4, remote router ID 2.2.2.2 BGP state = Established, up for 00:25:26 Last read 00:00:01, last write 00:00:01, hold time is 30, keepalive interval is 10 seconds |
（2）但从抓包情况看，open报文中，只有holdtime参数，并不包含keepalive参数，如图4所示。那么keepalive数值如何得出？
1) 为验证，调整R1和R2的timer参数，分别为：R1，keepalive 10秒，hold time 40秒。R2，keepalive 30秒，hold time100秒。此时两端协商后，由于只交互了hold time。R1的实际timer为，keepalive 10秒，hold time 40秒。R2的实际timer为，keepalive 13秒，hold time 40秒，keepalive数值为（最低的hold time值）/3的取整，如表5所示。
2) 为进一步验证，继续调整R1和R2的timer参数，分别为：R1，keepalive 10秒，hold time 20秒。R2，keepalive 30秒，hold time100秒。两端协商后，R1的实际timer为，keepalive 6秒，hold time 20秒。R2的实际timer为，keepalive 6秒，hold time 20秒，如表6所示。
对比两组实验结果，基本确定：
1) BGP邻居在交互open报文中，只包含holdtime字段，双方会取两者之间的最低值。
2) 但在本端keepalive数值确定中，两端均必须满足（hold time数值）≥（keepalive数值）*3。在配置的keepalive数值不满足此条件时，使用（最低的hold time值）/3的取整作为实际值。
表5 BGP邻居timer情况（二）
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R2#sh ip bgp nei 1.1.1.1 BGP neighbor is 1.1.1.1, remote AS 1, internal link BGP version 4, remote router ID 1.1.1.1 BGP state = Established, up for 00:00:17 Last read 00:00:07, last write 00:00:07, hold time is 40, keepalive interval is 13 seconds Configured hold time is 100, keepalive interval is 30 seconds !!!!在R1上查看R2的邻居状态情况 R1#sh ip bgp nei 2.2.2.2 BGP neighbor is 2.2.2.2, remote AS 1, internal link BGP version 4, remote router ID 2.2.2.2 BGP state = Established, up for 00:00:06 Last read 00:00:06, last write 00:00:05, hold time is 40, keepalive interval is 10 seconds Configured hold time is 40, keepalive interval is 10 seconds |
表6 BGP邻居timer情况（三）
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R1#sh ip bgp nei 2.2.2.2 BGP neighbor is 2.2.2.2, remote AS 1, internal link BGP version 4, remote router ID 2.2.2.2 BGP state = Established, up for 00:00:09 Last read 00:00:03, last write 00:00:02, hold time is 20, keepalive interval is 6 seconds Configured hold time is 20, keepalive interval is 10 seconds !!!!在R2上查看R1的邻居状态情况 2#sh ip bgp nei 1.1.1.1 BGP neighbor is 1.1.1.1, remote AS 1, internal link BGP version 4, remote router ID 1.1.1.1 BGP state = Established, up for 00:00:33 Last read 00:00:00, last write 00:00:02, hold time is 20, keepalive interval is 6 seconds Configured hold time is 100, keepalive interval is 30 seconds |
