RIP协议:发送路由更新之前跳数加1

今天上课的时候有一个问题，询问老师得到的结果和书上看到的不一样，做个实验验证一下。

考虑一个拓扑：

3台路由，4个网段，运行RIP v1协议，在R1上debug ip rip查看路由更新。

Router1#debug ip rip
RIP protocol debugging is on
*Mar  1 00:11:02.355: RIP: sending v1 update to 255.255.255.255 via FastEthernet1/0 (192.168.1.254)
*Mar  1 00:11:02.355: RIP: build update entries
*Mar  1 00:11:02.355:   network 172.16.0.0 metric 1
*Mar  1 00:11:02.355:   network 172.17.0.0 metric 2
*Mar  1 00:11:02.359:   network 192.168.2.0 metric 3
*Mar  1 00:11:02.831: RIP: received v1 update from 172.16.0.2 on FastEthernet0/0
*Mar  1 00:11:02.831:      172.17.0.0 in 1 hops
*Mar  1 00:11:02.835:      192.168.2.0 in 2 hops
*Mar  1 00:11:08.403: RIP: sending v1 update to 255.255.255.255 via FastEthernet0/0 (172.16.0.1)
*Mar  1 00:11:08.403: RIP: build update entries
*Mar  1 00:11:08.403:   network 192.168.1.0 metric 1
*Mar  1 00:11:29.791: RIP: received v1 update from 172.16.0.2 on FastEthernet0/0
*Mar  1 00:11:29.791:      172.17.0.0 in 1 hops
*Mar  1 00:11:29.795:      192.168.2.0 in 2 hops

从debug信息来看，RIP协议是在路由更新发送出去的时候就将跳数+1了，而不是收到的时候。