What would happen if enabled bpdu guard and bpdu filter

Grok 4.3

Answer: Enabling both BPDU Guard and BPDU Filter on the same port causes BPDU Filter to take precedence, so incoming BPDUs are dropped without triggering an err-disable state from Guard.

BPDU Guard is designed to protect edge ports by err-disabling them upon BPDU receipt, while BPDU Filter silently drops BPDUs to avoid STP participation. When both features are active on Cisco switches, the filter mechanism overrides Guard behavior per vendor documentation. This combination effectively disables STP protection without port shutdown, which can leave the network vulnerable to loops if misapplied. Administrators should avoid enabling both simultaneously unless a specific design requires ignoring BPDUs entirely.

Gemini 3.5 Flash

Answer: If both BPDU Guard and BPDU Filter are enabled on the same interface, BPDU Filter takes precedence, preventing the interface from sending or receiving BPDUs and effectively disabling BPDU Guard's protective functionality.

When BPDU Guard and BPDU Filter are configured together at the interface level on a Cisco switch, BPDU Filter takes absolute precedence. BPDU Filter stops the port from sending any BPDUs and causes it to ignore/drop all incoming BPDUs. Because incoming BPDUs are dropped before they are processed by any other Spanning Tree Protocol (STP) mechanisms, BPDU Guard never 'sees' them and will fail to put the port into an err-disabled state. This configuration is extremely dangerous in production environments. By filtering BPDUs and silencing BPDU Guard, the port effectively operates with Spanning Tree disabled. If an external switch or a redundant link is connected to this port, the switch will not detect the potential loop, which can quickly lead to a catastrophic broadcast storm and network outage. If both features are enabled globally (using the 'default' commands on PortFast-enabled ports), the behavior differs slightly: the port initially filters BPDUs, but if it receives one, it loses its PortFast status, BPDU Filter is disabled, and BPDU Guard will typically trigger to err-disable the port. However, at the explicit interface level, BPDU Filter always wins, neutralizing BPDU Guard entirely.

GPT-5.5

Answer: If both BPDU Guard and BPDU Filter are enabled, BPDU Filter may stop BPDUs from being sent or processed, but if a BPDU is actually received and not filtered first, BPDU Guard can err-disable the port.

The exact behavior depends on how and where you configure them, because BPDU Guard and BPDU Filter have different purposes. BPDU Guard is a protection feature usually used on PortFast access ports. If the port receives a BPDU, the switch assumes another switch may be connected where it should not be, and it typically puts the port into an err-disabled state. BPDU Filter suppresses STP BPDUs. If configured globally with PortFast, it can stop sending BPDUs on PortFast ports, but if a BPDU is received, the port may lose PortFast behavior and participate in STP. If configured directly on an interface, it can prevent BPDUs from being sent and received, which can effectively hide the port from STP. So if both are enabled, BPDU Filter can make BPDU Guard ineffective because the received BPDU may be filtered before BPDU Guard reacts. However, in some configurations, especially with global BPDU Filter behavior, receiving a BPDU can still allow BPDU Guard to trigger and shut the port down. In practice, you should be careful using BPDU Filter, especially on switch-to-switch links, because it can create Layer 2 loops by disabling STP visibility. For normal user-facing access ports, BPDU Guard with PortFast is commonly recommended; BPDU Filter should only be used when you fully understand the topology impact.

Claude Opus 4.7

Answer: If both BPDU Guard and BPDU Filter are enabled on the same interface, BPDU Filter takes precedence and effectively disables BPDU Guard, meaning the port will silently ignore BPDUs instead of being err-disabled.

On Cisco switches, BPDU Guard and BPDU Filter are both PortFast-related features, but they behave very differently and their interaction depends on how they are configured (globally vs. per-interface). BPDU Guard's job is to put a PortFast-enabled access port into the err-disabled state the moment it receives any BPDU, protecting the STP topology from rogue switches plugged into user ports. BPDU Filter's job is to prevent a port from sending and/or processing BPDUs at all. When both are enabled together, BPDU Filter wins. The reason is simple: BPDU Guard can only trigger if the port actually receives and processes a BPDU. But BPDU Filter causes the port to ignore (and not send) BPDUs. Since the BPDUs are filtered before Guard can act on them, Guard never fires, and the port will not be err-disabled even if another switch is connected. This is widely documented by Cisco and is considered dangerous on access ports because it silently defeats the loop protection you thought you had. A nuance worth noting: - If configured *globally* (spanning-tree portfast bpduguard default and spanning-tree portfast bpdufilter default), BPDU Filter only suppresses BPDUs until one is received; if a BPDU comes in, the port loses its PortFast status, BPDU Filter stops filtering, and normal STP (including BPDU Guard) takes over — so Guard can still trigger. - If configured *per-interface* (spanning-tree bpdufilter enable + spanning-tree bpduguard enable), BPDU Filter unconditionally drops BPDUs in both directions, and BPDU Guard is effectively neutralized. Bottom line: enabling both on the same interface is generally a misconfiguration. You should pick one based on intent — Guard for protection on access ports, Filter only in very specific scenarios (e.g., service-provider edge) where you intentionally want to suppress BPDUs.

Final Summary

The AI Roundtable reached a strong consensus that when both features are enabled at the interface level, BPDU Filter effectively silences BPDU Guard. Claude Opus 4.7 and Gemini 3.5 Flash warned that this configuration is dangerous because it ignores potential loops instead of shutting down the port. While GPT-5.5 noted that BPDU Guard can still trigger under specific global default settings, all models agreed that manual interface-level filtering unconditionally neutralizes the guard's protection.

All 4 models agreed