免MSTP链路恢复了，端口仍处于关闭状态。反正MSTP线路故障不恢复的话就算UP了，也收不到路由更新。
action 4.0 cli command \! end R2： hostname R2 !
boot-start-marker boot-end-marker !
enable secret 5 $1$XJXH$eN1O2.Eyv1T5GiWj/M2Rb/ !
no aaa new-model
no network-clock-participate wic 1 dot11 syslog ip source-route ! ! ! ip cef
no ip domain lookup
multilink bundle-name authenticated ! ! ! ! ! ! !
controller E1 0/1/0 line-termination 75-ohm channel-group 0 unframed !
controller E1 0/1/1 ! !
track 1 ip sla 1 ! ! ! !
interface Loopback0
ip address 56.26.0.1 255.255.255.0
!
interface Loopback1
ip address 56.26.128.1 255.255.255.0 !
interface FastEthernet0/0
ip address 56.16.234.2 255.255.255.252 speed 100 full-duplex !
interface FastEthernet0/1 speed 100 full-duplex !
interface Serial0/1/0:0
ip address 56.16.244.2 255.255.255.252 !
router rip version 2
offset-list 0 in 3 FastEthernet0/1 offset-list 0 out 3 FastEthernet0/1 network 56.0.0.0 no auto-summary
!
ip forward-protocol nd ! !
ip http server
no ip http secure-server
ip route 56.16.234.1 255.255.255.255 FastEthernet0/0 ! ip sla 1
icmp-echo 56.16.234.1 frequency 5
ip sla schedule 1 life forever start-time now disable-eadi ! !
control-plane ! !
line con 0 line aux 0 line vty 0 4 password cisco
login !
scheduler allocate 20000 1000 event manager environment 1 event manager applet rip_track_2 event track 1 state down action 1.0 cli command \action 2.0 cli command \action 3.0 cli command \action 3.1 cli command \action 3.2 cli command \action 4.0 cli command \! end
当两个交换机之间的连线拔掉之后，track 1 可以发现56.16.234.2无法ping到，SLA状态转为down，触发了EEM，执行EEM定义的语句。 *Jan 9 05:58:40.911: %TRACKING-5-STATE: 1 ip sla 1 state Up->Down
*Jan 9 05:58:40.995: %SYS-5-CONFIG_I: Configured from console by on vty0 (EEM:rip_track_1) *Jan 9
05:58:42.963:
%LINK-3-UPDOWN:
Interface
FastEthernet0/0, changed state to up
再查看路由表时，路由已经是从S0/1/0：0学习到的。在配置中的探测频率与延时情况下，粗略判断路由收敛的时间在5秒左右，比自行收敛快数十倍。
其中有一个需要注意到地方是配置中的那条主机路由：ip route 56.16.234.2 255.255.255.255 FastEthernet0/0
用于确保探测对端地址的ICMP报文只能从F0/0发出，避免ICMP探测报文从其他路径到达目标，造成误判。
初步使用了一下EEM，觉得这个功能非常强大，值得进一步学习。