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Auto-Preemption

date_range 24-Feb-25

Enable auto-preemption to improve load-balancing in your Mist Edge tunnels.

Auto-preemption is a useful feature to enable on your Mist Edge tunnels. This feature, which runs in the Mist cloud, ensures that whenever traffic becomes unbalanced across a cluster, it is rebalanced across the available Mist Edge devices.

How Traffic Becomes Unbalanced in a Mist Edge Cluster

Normally, when you create a Mist Edge cluster, traffic is load-balanced across the Edge appliances. For example, if you have ME1, ME2, and ME3, traffic is load-balanced across all three.

However, there are several situations when traffic becomes unbalanced. For example:

  • ME1 is rebooted. Traffic is rebalanced across the remaining two appliances. Even after ME1 comes online, the APs continue to use the other appliances.

  • ME2 is unreachable. During this process, traffic is rebalanced across the remaining two appliances. Even after connectivity is restored, the APs continue to use the other appliances.

  • A new appliance, ME4, is added to the cluster. The APs continue to use their preferred appliances (M1, M2, or M3).

In such situations, you can manually preempt or disconnect the tunnels by bouncing the AP tunnels through the API or the Juniper Mist portal. When the tunnel is reestablished, traffic will load-balance across all reachable appliances.

Benefits of Auto-Preemption

When you enable auto-preemption, this cloud-based service periodically checks for imbalances. This service determines which Edge appliances are up and healthy, and then slowly rebalances the traffic across all available Edge appliances in the cluster. There is no impact to the AP tunnels on the correct primary Edge. This is done seamlessly so that the clients on the AP are not deauthenticated.

Enabling Auto-Preemption

You can enable auto-preemption in the tunnel settings page for your site-level and organization-level tunnels. For help getting started with tunnels, see Deploy Juniper Mist Edge at the Organization Level or Deploy Mist Edge at the Site Level.

In the Auto Preemption section of the Mist Tunnel settings, click Enabled, and then enter the schedule. Typically, you’ll configure this feature to run at a time and day when your network is the least busy.

Figure 1: Auto-Preemption for an Organization-Level Tunnel Organization-Level Auto Preemption
Figure 2: Auto-Preemption for a Site-Level Tunnel Site-Level Auto Preemption Settings
Note:
  • The Time of Day/Day of Week option is likely to cover most use cases. The Every 15 Minutes setting might be used if connectivity between the APs and the Mist Edge cluster is jittery and the secondary edge is in an L3-separated data center with a different IP schema. However, it's generally preferable to set a maintenance window by using the Time of Day/Day of Week option.

  • If you're configuring the site-level option, the schedule is in the site's time zone.

  • For a site-level tunnel, the time of day is in the site's time zone.

  • For an organization-level tunnel, the time of day is in UTC time.

AP Auto-Preemption Skipped Events

There are certain conditions when the auto-preemption cannot be completed. For example:

  • The tunnel service is not running

  • The tunnel IP is not reachable

  • Upstream ports are down

If auto-preemption is skipped, the service will run again at the next scheduled day and time.

You can see information about skipped events on the Insights page.

Skipped Auto-Preemption Event on the Insights Page

To find these events, go to Monitor > Service Levels, and click the Insights button. From the context menu, select the Mist Edge that you want to monitor. Also select the time period to view. The page reloads with the relevant information, including all Mist Edge Events.

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