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Optimizing and Protecting Spanning Tree

Optimizing STP

Left to defaults, 802.1d (plain old STP) can take a very long time to converge.  For example, when a root switch fails, a switch must wait Maxage (20 seconds) before convergence can even begin.  Then, the newly forwarding ports must wait 2 x Forward Delay (15 seconds) to transition through the listening and learning states before they can begin to actually start forwarding.  This is a total of 50 seconds - a noticeable network hit.

Enhancements have been added over time to address this, such as PortFast, UplinkFast, and BackboneFast.

PortFast

This Cisco-proprietary feature allows a port to immediately transition to forwarding state once it is physically up (powered on and plugged in).  It does this by skipping the listening and learning states.  This should only be enabled on access ports.  If a switch is connected to a port with PortFast enabled, loops may occur.  For this reason, it is a good idea to enable Bridge Protocol Data Unit (BPDU) Guard and Root Guard when using PortFast.

UplinkFast

UplinkFast improves convergence by providing alternate root ports (RPs) for immediate transition in case of a failure of the current RP.  Continue reading