An EX3400 Virtual Chassis running Junos 23.4R2-S7 experienced a temporary loss of ISP connectivity. The outage occurred and recovered a few minutes later.

Analysis of the available STP logs and syslog messages indicates that the outage correlated with an STP topology change and temporary root bridge reconvergence.

Temporary loss of connectivity toward the ISP.

No alarms or interface errors were observed.

No interface flaps were observed on the QFX uplink or ISP-facing interface.

STP Topology Change Notifications (TCNs) were observed on the QFX-facing uplink.

The local switch generated an STP topology change.

The STP root bridge changed from the QFX switch to the local EX3400 VC.

Root protection was subsequently triggered on the customer-facing interface XXX.

The original QFX root bridge was learned again later, and the EX3400 reverted to the original root.

The EX3400 initially had the QFX switch as the STP root: 4096:XXX

During the reported outage, the EX3400 temporarily stopped learning/reaching the original root bridge and reconverged toward the remaining STP topology.

As a result, the local EX3400 temporarily became the STP root: 61440:XXX

The EX3400 did not become root because it had a superior bridge priority. The original root had a priority of 4096, while the temporary local root had a priority of 61440. This indicates that the original root was temporarily unavailable from the EX3400's STP perspective.

During reconvergence:

1. The QFX-facing interface XXX was selected as the root port.
2. STP topology changes propagated toward the customer-facing interface XXX.
3. Root protection was triggered.
4. Additional TCNs were generated.
5. Once the original root became reachable again, the EX3400 reverted to the original root.

The available logs do not indicate a configuration change or physical interface failure as the initiating event.

To prevent the EX3400 from becoming the STP root in this topology, configure the QFX-facing interfaces toward the EX3400 as no-root-port.

This helps ensure that the intended QFX switch remains the STP root and prevents the EX3400 from influencing root selection during STP reconvergence.