Summary: UAC (Uplink Auto Config) is the subsystem on hybrid IOS-XE Catalyst switches that automatically identifies and selects the uplink interface for Meraki Dashboard connectivity, eliminating the need for manual uplink configuration. UAC scores candidate interfaces on a 0–12 scale. Automatic fallback to the preferred uplink was introduced in 17.15.4.1 but was not reliably functional; 17.15.5 is where the behaviour became fully reliable, adding a working automatic fallback checkbox and replacing ping-based probing with ARP only.
On Catalyst switches running in Meraki cloud-managed (hybrid) mode, UAC replaces manual uplink configuration. It continuously monitors available interfaces, scores them, and selects the highest-scoring one to maintain connectivity to the Meraki Dashboard. If the active uplink fails, UAC fails over automatically to the next best candidate.
UAC identifies active VLANs by monitoring data plane statistics — a VLAN is considered active once it receives more than 60 frames in a 2-minute window. UAC then tests gateway reachability on those VLANs to score each candidate interface.
UAC assigns each candidate interface a score in the range 0–12. The score is calculated from a combination of:
catalyst.meraki.com (17.15.4.1 / 17.18.1 / 17.18.2 only)If gateway ARP fails three consecutive probes, the interface score is set to 0, and UAC fails over to the interface with the next highest score.
In IOS-XE 17.15.5 and later, UAC uses ARP probes only (ping tests are removed), and uplink health is checked continuously rather than at fixed intervals.
From the Meraki dashboard, an administrator can designate a preferred uplink — the interface UAC should use as its first choice. The behaviour of this setting differs significantly by IOS-XE version:
| IOS-XE Version | Preferred Uplink Behaviour |
|---|---|
| 17.15.4.1 | Introduced automatic fallback to preferred uplink, but the feature is not reliably functional. UAC starts on the preferred uplink, but once it moves away it may not return under all conditions — only a complete failure of the current uplink's Dashboard connectivity guarantees a switch back. 17.18.1 and 17.18.2 carry the same limitation. |
| 17.18.1, 17.18.2 | Same behaviour as 17.15.4.1 — automatic fallback present but unreliable. |
| 17.15.5 and later | Automatic fallback made fully reliable. UAC always starts on the preferred uplink after a reboot. The dashboard checkbox "Automatically fallback to preferred uplink when available" works consistently. Ping-based probing replaced with ARP probes only, which improves detection reliability. |
The unreliable behaviour in 17.15.4.1–17.18.2 was an intentional design trade-off to prevent tunnel interface flapping to Dashboard. 17.15.5 revised the probing mechanism (ARP only) in a way that made the automatic return safe to enable reliably without risking instability.
For devices running 17.15.4.1, 17.18.1, or 17.18.2, the recommended fix is to upgrade to 17.15.5 or later. Where an upgrade is not immediately possible, two options exist:
For production deployments, standardise on IOS-XE 17.15.5 unless a specific feature requires a later train (17.18.x or 26.x). It is the release where the most significant UAC reliability improvements landed — ARP-only probing, dependable preferred uplink fallback, and correct reboot behaviour. Later trains carry forward these improvements, but 17.15.5 is the earliest version where UAC can be considered fully production-ready.
⚠️ CS Firmware Migration
Migrating from CS firmware to IOS-XE 17.15 is not easily reversible. A factory reset and TAC involvement may be required to go back. Treat this as a planned change, not a routine upgrade.
Introduced in IOS-XE 17.18.2, the UAC Allow List restricts UAC to a specific set of interfaces rather than considering all active VLANs. This is configured in the dashboard alongside corresponding static routes and candidate uplink designation, and is useful in environments where strict control over which interfaces UAC may use is required.
show uac uplink
show uac uplink db
These commands display the current active uplink, the configured preferred uplink, interface scores, and — when the Allow List is enabled — any candidate interfaces in the allow list.
When UAC selects the wrong uplink or the switch cannot reach the Meraki Dashboard, work through these checks in order.
show uac uplink.