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In the production, is there an easy way to do the auto failover for Internet?

It is very common to have two Internet circuits for a campus network. Usually one is the primary and another one is the backup link. You can enable the dynamic route between edge layer3 devices and ISP devices, such as BGP, to achieve the Internet link failover. But for the small campus which doesn’t want to enable the BGP protocol, you can configure the floating static route with SLA tracking feature.

For example ISP1 is the primary router for Internet and ISP2 is the backup router for Internet.

 
On the Edge Router
Configure the SLA using Ping to check the connectivity between Edge Router and ISP1 and also configure two default routes.

  • ip route 0.0.0.0 0.0.0.0 100.1.20.1 track 10: if the connectivity of track 10 is up, this default route will be active. It has default AD value 1.
  • Ip route 0.0.0.0 0.0.0.0 100.1.10.1 50: this default route has been configured with AD value 50. it will not be showed in routing table if the above one default is active

Once SLA has been started on the Edge router, the Edge router will send icmp echo (ping) frequently to detect the connectivity between ISP1 and itself.
The default time for SLA:

Cisco recommends these default values when you configure the IP SLA:

  1. Threshold(millisecs): 5000
  2. Timeout(millisecs): 5000
  3. Frequency(secs): 60

When Edge router can ping ISP1 (100.1.20.1), the default route will point to ISP1.
 


 
When the link between Edge router and ISP1 is down, the SLA will detect it. The default route will point to ISP2
 

 
Once the link between ISP1 and Edge router is up, SLA will detect it via icmp echo. But it will wait for the timer and frequency . You can tune the timer and frequency based on your network environment. The default will point to ISP1 again.
 

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