Spanning Tree Protocol, the loop-prevention mechanism that keeps Layer 2 networks stable. Articles tagged STP cover port roles and states, RSTP and MST, root bridge configuration, BPDU Guard and Root Guard hardening, and Cisco Catalyst troubleshooting.
A Linux bridge with STP on sent one 802.1D BPDU into a PortFast access port on IOL-L2. BPDU Guard logged SPANTREE-2-BLOCK_BPDUGUARD and err-disabled the port, proven by the capture and syslog. Storm control was rejected by the parser.
Calculate root ports on six switches: path cost settles SW4, neighbor bridge ID settles SW5 and neighbor port ID settles SW6. Check the real outputs, then change one cost to move SW4's root port.
Five Layer 2 faults built and broken in a CML lab, ticket style: VLANs, trunks, and a spanning tree quietly blocking the port you need. Real show output, real syslog messages, and the real fix for each one.
A link that transmits but cannot receive never hears the BPDU telling it to block, and spanning tree loops. UDLD detects unidirectional links and shuts the port first. Covers normal and aggressive mode with real CML output.
An MST core touching a Rapid-PVST access layer is one of the most misunderstood seams in Layer 2. Build the boundary in a CML lab, see the CIST at work, and deliberately trigger the PVST simulation inconsistency so you can recognize it.
PortFast skips the 30-second spanning tree wait so a host gets an IP the moment it plugs in. This post covers where it belongs, where it can take down a network, and the two guard features that keep it safe.
How EtherChannel hides bundled links from STP, the bundling protocols (LACP, PAgP, on), the dangerous failure modes, and why LACP active on both sides is the only safe configuration.
Classic 802.1D STP vs 802.1w RSTP. The convergence differences (30s vs sub-second), the port state and role changes, and the one-command Cisco migration from PVST+ to Rapid PVST+.
This is the capstone article: a comprehensive pre-deployment checklist covering all STP hardening requirements. Organized by switch role, this checklist ensures your campus network STP implementation is secure, predictable, and ready for production traffic.
Troubleshooting STP requires knowledge of dozens of show and debug commands. This article provides a reference for every relevant command in Cisco IOS XE, with annotations explaining what each output means and when to use it.
The STP root bridge and the HSRP/VRRP active gateway should reside on the same switch. When they don't, traffic flows upstream to one switch and returns via another, wasting bandwidth and causing asymmetric path problems. This article covers alignment strategy and verification.
In a three-layer campus, STP topology must align with the physical hierarchy. This article shows how to place root bridges, design inter-layer redundancy, and troubleshoot anomalous topologies created by misconfiguration across access, distribution, and core layers.