STP (Spanning Tree Protocol) is a great invention by Radia Perlman. The protocol was designed to ensure that Layer 2 Ethernet network is loop-free. “Algorhyme” is a poem by Radia Perlman that pretty much summarizes what STP is. While this is a great invention, it needed some improvements. Say hello to Cisco’s STP Toolkit.
Cisco STP Toolkit is a collection of STP extensions that improves the performance of the original IEEE 802.1D STP algorithm. Here are the extensions included in the STP Toolkit:
- PortFast – Mostly used in access ports, but can be used in trunk ports. If you choose to enable PortFast on trunk ports, make sure you’re not creating a loop. This extension causes an access port or trunk port to go to Forwarding STP state immediately, basically skipping the listening and learning state.
- UplinkFast – Used in uplink ports to speed up STP convergence after a direct failure.
- BackboneFast – Speeds up STP convergence after an indirect failure. This extension needs to be enabled on all network devices to take advantage of the feature.
- Loop Guard – Helps prevent Layer 2 loop when there is an unidirectional link failure. It prevents an alternate or root port from becoming a designated port if it stops receiving BDPUs, it transitions to loop-inconsistent state.
- Root Guard – Prevents external switches becoming a root. These are normally enabled on ports connecting to downstream switches. When a superior BPDU is received from an interface where root guard is enabled, the switch port will transition to root-inconsistent state.
- BPDU Guard – When PortFast is enabled on a port, this STP extension helps prevent bridging loops by transitioning a switch port to err-disabled upon receiving a BPDU.
- BPDU Filter – This STP extension prevents PortFast-enabled ports from sending and receiving BPDUs – effectively disabling STP at the edge which can lead to bridging loops. This is not a recommended configuration per the authors of CCDP ARCH Self-Study Guide.
- UDLD (Unidirectional Link Detection) – This STP extension prevents bridging loops by monitoring the fiber optic and/or twisted-pair links and detecting if a one-way or unidirectional communication exists. If it detects a unidirectional communication, it will shut down the interface and there will be a system alert.
Some Cisco documentation include UDLD and BPDU Filter and some do not. Just for the sake of completion, I included them both here.
I hope this has been helpful and thank you for reading!
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