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Once the forwarding loop starts, it congests the lowest-bandwidth links along its path.

A forwarding loop (such as an STP loop) occurs when no port in a redundant topology is blocked, and traffic is forwarded in circles indefinitely. The purpose of STP is to block redundant ports based on an STP algorithm, to resolve redundant physical topology into a tree-like topology. Issues with most STPs have these three problems:Įxcessive flooding due to a high rate of STP Topology Changes (TC).īecause a bridge has no mechanism to track whether a certain packet is forwarded multiple times (for example, an IP Time to Live is used to discard traffic that circulates for too long in the network, only one path can exist between two devices in the same Layer 2 (L2) domain. There are specific commands which apply to the Catalyst 6500/6000 only however, you can apply most of the principles to any Cisco Catalyst switch that runs Cisco IOS software. This document describes the how to use Cisco IOS® software to troubleshoot issues with Spanning Tree Protocol (STP).