How long does it take for a port to transition from the STP blocking state to the forwarding state by default?
- 2 seconds
- 10 seconds
- 25 seconds
- 50 seconds
- 70 seconds
It usually takes 50 seconds for a port to transition from the blocking state to the forwarding state in STP. This delay is a function of the default settings for the forward-delay and max-age settings. The max-age delay is 20 seconds by default, and is used to transition from the blocking to the listening state. The forward-delay setting is 15 seconds by default. This timer is used in the transition from the listening to learning states, and again for the transition from the learning to the forwarding state. These timers give STP time to gather the correct information about the network topology. While they can be modified to make convergence more efficient, the default settings work for most networks. To change the timers on all switches in the VTP domain, change the timer settings on the root bridge and the changes will be forwarded to the other switches.
To prevent switching loops, spanning tree transitions each port through several states whenever there is a change in the network topology. Each state is briefly defined as follows:
- Blocking: In the blocking state, a port does not forward frames, learn information, or send information. A forwarding port is placed in the blocked state when the port senses an absence of BPDUs, which are sent in the interval defined by the hello timer (two seconds by default). If the blocked port does not detect a BPDU for the length of time defined in the max-age setting (20 seconds by default), the port will transition into the listening state.
- Listening: In the listening state, a port receives traffic but does not send information. This is the first transitional state after the blocking state. No user data is forwarded at this time, but the switch is very busy. It is during this stage that the switch participates in the election of the root bridge, the designation of root ports on the non-root bridges, and the selection of designated ports on each segment. Ports that are designated or root ports will transition to the learning state after the time defined in the forward delay (15 seconds by default) has elapsed.
- Learning: In the learning state, a switch port can add the MAC addresses that it has learned into its address table, but cannot forward user data. The switch port will remain in this state until the amount of time defined in the forward-delay setting has elapsed (15 seconds by default), at which time it will transition into the forwarding state.
- Forwarding: In the forwarding state, a port is actively forwarding packets. It will remain in the forwarding state until it does not detect a BPDU within the defined hello time, at which time the port is placed in the blocking state and the process starts again.
NOTE: One of the issues that can adversely affect the operation of STP is a duplex mismatch between the NICs on either end of a link between two switches. While this causes more of a performance problem than a loss of the link, the intermittent nature of the outage can cause one of the other links on the switch to transition into a forwarding state, as it may interpret this as a loss of connectivity. If one of the other links switches to forwarding and the link with the duplex mismatch comes back online (which could happen quickly), it can create a switching loop.
Layer 2 Technologies
Configure and verify spanning tree