自动切换策略，具体如下
监视地址：1.1.1.1
轮询时间：30s；超时时间：1000ms
up
/ip firewall nat set [/ip firewall nat find comment ="test" to-addresses=x.x.x.x
/ip firewall address-list set [/ip firewall address-list find comment="test"] disabled=no
down
/ip firewall nat set [/ip firewall nat find comment ="tes"] to-addresses=y.y.y.y
/ip firewall address-list set [/ip firewall address-list find comment="test"] disabled=yes
监视地址：2.2.2.2
轮询时间：30s；超时时间：1000ms
up
/ip firewall nat set [/ip firewall nat find comment ="test1"] to-addresses=a.a.a.a
/ip firewall address-list set [/ip firewall address-list find comment="test1"] disabled=yes
down
/ip firewall nat set [/ip firewall nat find comment ="test1"] to-addresses=b.b.b.b
/ip firewall address-list set [/ip firewall address-list find comment="test1"] disabled=no