This command causes the switch to begin forwarding immediately, even though the ports continue to participate in Spanning Tree. It shouldn't be configured on any ports that go to other switches, hubs, or bridges, however, because the immediate forwarding on such a link could render the network prone to a forwarding loop outage. Sometimes network users complain of slowness or bad application performance, problems that are often attributed to the network. For such scenarios, consider the entire situation. Check that every step along that path is correct and running cleanly.
Many major problems stem from minor ones such as duplex mismatch.
Use the show port command to see if Catalyst ports are accepting errors, and use the show mac command to monitor how many frames have been transmitted and received. Often the switches, and indeed the entire network, are running smoothly, and the problem lies elsewhere. Looking at everything and ruling things out can speed identification of the cause. VLAN Assignment VLANs create separate Layer 2 networks within the same physical infrastructure. However, VLANs only work when all switches in the network agree on which VLANs exist.