環境說明：兩條專線接serial,一條備援adsl接fastethernet
偵測1.1.1.1這個loopback ip,假如偵測不到就會刪掉此路由
source-interface記得加上去,不然會有路由震盪問題
ps：如果沒有指定source-interface,當serial up但是偵測不到ip時,
路由會先切換到備援線路,備援線路up時1.1.1.1就ping的到,
但是因為沒有指定source-interface,所以serial路由又會恢復,
但是路不通,此時便會一下子通一下子不通.
ip sla monitor 1
type echo protocol ipIcmpEcho 1.1.1.1 source-interface Serial0/0/0
exit
ip sla monitor schedule 1 life forever start-time now
track 123 rtr 1 reachability
exit
ip sla monitor 2
type echo protocol ipIcmpEcho 1.1.1.1 source-interface Serial0/0/1
exit
ip sla monitor schedule 2 life forever start-time now
track 124 rtr 2 reachability
exit
ip route 0.0.0.0 0.0.0.0 Serial0/0/0 track 123
ip route 0.0.0.0 0.0.0.0 Serial0/0/1 track 124
ip route 0.0.0.0 0.0.0.0 2.2.2.2 200