(This documentation tool is very useful. It is also a relatively common demand encountered by customers at present. Hope it works for everyone. When the network connection is normal, the network segment 1 (172.16.1.0/24) reaches 6.6.6.6 through ISP_A, and the network segment (172.16.2.0/24) reaches 6.6.6.6 through ISP_B. When one of the links of ISP_A or ISP_B fails , can achieve their own switching, has achieved the purpose of backup.)