IP SLA - Route Flapping Problem

IP SLA can be used to perform reliable static routing. However, there is the possibility of such a scenario causing route flapping. Route flapping between primary and secondary routes, is a common challenge in network configurations. Here's an overview of the situation and a potential solution:

Problem Overview

  1. IP SLA Operation: You're using IP SLA to monitor connectivity to a specific server (i.e. 5.5.5.5) via ICMP Echo.
  2. Primary Route Removal: If the IP SLA operation fails (indicating connectivity issues), the primary route to the internet is removed from the routing table.
  3. Backup Route Activation: A backup default route is then installed to restore connectivity.
  4. Route Flapping: Once the backup route is in place, the IP SLA operation may detect that connectivity to 5.5.5.5 is restored, causing the primary route to be reinstalled. This can lead to a cycle of the primary route being installed and uninstalled, known as "route flapping".

Potential Solution: Introducing Delay

To mitigate this issue, you can introduce a delay in the IP SLA configuration to prevent immediate reinstatement of the primary route. This can be done by configuring a delay for the down and up states of the track object:

  1. Delay for Down State: Introduce a delay for the track object to go down after the IP SLA operation fails. This ensures that a temporary loss of connectivity doesn't immediately trigger a route switch.

  2. Delay for Up State: Introduce a delay for the track object to come back up after the IP SLA operation is successful again. This delay helps to ensure that the connectivity is stable before switching back to the primary route.

Additional Considerations

Other solutions include:

  • Multiple Targets for IP SLA: Instead of relying on a single destination for IP SLA, consider using multiple targets. This approach reduces the likelihood that transient issues with a single destination will cause route flapping.
  • Tuning Timers: Adjust the delay timers and IP SLA operation frequency based on the specific requirements and behavior of your network.
  • Logging and Monitoring: Ensure that you have proper logging and monitoring in place to track the behavior of your IP SLA operations and route changes. This data can be invaluable for adjusting your configuration and understanding the network's behavior.
  • Change the Monitoring Target: Instead of monitoring a single IP like 5.5.5.5, you might consider monitoring a device or service that is indicative of overall Internet health or critical services. This might prevent the routing loop if the single IP becomes unreachable for reasons not indicative of a primary route failure.
  • Policy-Based Routing (PBR): Use PBR to define specific conditions for traffic routing that are more complex than simple reachability.

Implementing these strategies should help in creating a more stable IP SLA configuration and avoid the route flapping issue you're concerned about.

https://networklessons.com/cisco/ccie-enterprise-infrastructure/reliable-static-routing-with-ip-sla

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