7-3. STP Convergence Tuning

· STP bases its operation on several timers. Usually, the default timer values are used for proper STP behavior. The defaults are based on a network diameter of seven switches but can be adjusted for faster convergence times.
- The Hello Timer triggers periodic hello messages to neighboring switches.
- The Forward Delay timer specifies the time a port stays in each of the listening and learning states.
- The MaxAge timer specifies the lifetime of a stored BPDU received on a designated port. After the timer expires, other ports can become designated ports.
· BPDUs are expected at regular intervals. If they are delayed beyond the lapse of an STP timer, topology changes can be triggered in error. This condition can be detected with the BPDU skewing feature.
· STP PortFast allows ports that connect to hosts or nonbridging network devices to enter the forwarding mode immediately when the link is established. This bypasses the normal STP port states for faster startup, but allows the potential for bridging loops to form.
· STP UplinkFast is used only on leaf-node switches (the ends of the ST branches), usually located in the access layer. The switch keeps track of all potential paths to the root, which are in the blocking state.
- When the root port fails, an alternate port is brought into the forwarding state without the normal STP port state progression and delays.
- When UplinkFast is enabled, the bridge priority is raised to 49152, making it unlikely to become the root bridge. All switch ports have their port costs increased by 3000 so that they won't be chosen as root ports.
- When an alternate root port comes up, the switch updates upstream switches with the new location of downstream devices. Dummy multicasts are sent to destination 01-00-0C-CD-CD-CD that contains the MAC addresses of stations in the bridging table.
· STP BackboneFast causes switches in the network core to actively look for alternate paths to the root bridge in case of an indirect failure.
- When used, this feature should be enabled on all switches in the network. Switches use a request-and-reply mechanism to determine root path stability, so all switches must be able to participate.
- BackboneFast can only reduce the convergence delay from the default 50 seconds (20 seconds for the MaxAge timer to expire, and 15 seconds in both listening and learning states) to 30 seconds.