For a stack to form, all member switches must be running compatible IOS versions that support the stacking feature. Mismatched or unsupported versions are a common cause for stack formation failures. While topology and priority can affect master election, the stack should still form; mismatched IOS versions will prevent it entirely.

Proper QoS configuration is essential for voice quality. The key is ensuring the switch is correctly classifying voice traffic (marked by the phone as DSCP EF/46) and placing it into a low-latency queue. Without this configuration, voice packets can be delayed or dropped during congestion, causing quality issues, regardless of VLAN connectivity.

While PortFast is recommended for access ports to speed up client connectivity, enabling it without BPDU Guard is a significant risk. If a user connects a switch to this port, the absence of BPDU Guard means the rogue switch could participate in STP, potentially causing a topology change or loop. BPDU Guard should always be paired with PortFast on user-facing ports.

Cisco’s Plug-and-Play (PnP) and Zero-Touch Provisioning (ZTP) workflows are designed for this scenario. When a new switch is added, it should use PnP to contact a provisioning server (like DNA Center), which identifies the switch by its serial number or other unique ID and pushes the correct, conflict-free configuration (including a unique management IP) to it.

This is the correct and secure configuration. It sets the port to trunk mode, specifies a non-default native VLAN (999 is often used as a ‘blackhole’ VLAN), explicitly allows only the necessary VLANs (10,20,30), and disables Dynamic Trunking Protocol (DTP) with ‘nonegotiate’ to prevent the port from negotiating a different mode, hardening it against VLAN hopping attacks.

A steady increment in ‘runts’ (frames that are smaller than the minimum 64-byte Ethernet frame size) is a classic symptom of a duplex mismatch. When one side is set to full-duplex and the other to half-duplex, collisions occur on the full-duplex side, which then transmits collision fragments (runts). This is a common issue when auto-negotiation fails or is disabled.

The power of Cisco DNA Center’s template-based provisioning is reusability and centralized management. The correct workflow is to edit the existing template (or create a new version of it) with the updated NTP server IP, then re-deploy this updated template to the target device group. This ensures a consistent, automated, and error-free update across all devices.

The standard and effective method is to place guest traffic in a dedicated VLAN. The security is then enforced at the Layer 3 boundary (the SVI for that VLAN) using an Access Control List (ACL). This ACL should permit traffic to the internet (e.g., to the default gateway and external DNS) and explicitly deny traffic to the internal corporate network subnets, providing simple but effective isolation.

A native VLAN mismatch occurs when the configured native VLAN (untagged VLAN) on two ends of an 802.1Q trunk are different. This causes traffic from one native VLAN to leak into another. This is a security concern because it can be exploited in VLAN hopping attacks, where an attacker sends double-tagged frames to jump from one VLAN to another, bypassing intended segmentation.

For a stack to form, all member switches must be running compatible IOS versions that support the stacking feature. Mismatched or unsupported versions are a common cause for stack formation failures. While topology and priority can affect master election, the stack should still form; mismatched IOS versions will prevent it entirely.0