Additional Spanning Tree Features

Spanning tree was originally designed for bridges with few ports. With the advent of Ethernet switches, some enhancements were made to spanning tree. These commonly seen enhancements helped make spanning tree more palatable by decreasing the time a host needs to wait for a port, and decreasing the convergence time in a Layer-2 network.


PortFast is a feature on Cisco switches that allows a port to bypass all of the other spanning tree states (see Figure 8-5, shown earlier) and proceed directly to the forwarding state. PortFast should be enabled only on ports that will not have switches connected. Spanning tree takes about 30 seconds to put a normal port into the forwarding state, which can cause systems using DHCP to time out and not get an IP address (on a Windows machine, a default IP address may be used). Enabling the PortFast feature on a port alleviates this problem, but you should be very careful when using this feature. If a switch were to be connected to a port configured with PortFast active, a loop could occur that would not be detected.

To enable PortFast on an IOS switch, use the spanning-tree portfast interface command. The switch will deliver a nice warning about the dangers of PortFast when you enable the feature:

Cat-3550(config-if)#spanning-tree portfast %Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators, switches, bridges, etc... to this interface when portfast is enabled, ...

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