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The previous section has said that each time the bridge is in the listening state, the default is to listen for 15 seconds, in fact, in general, STP is subject to 3 time in control, such as table 6-5.
Table 6-5. STP Timers
Timer |
Primary Purpose |
Default |
Hello time |
Time between sending of Configuration BPDUs by the Root Bridge |
2 Secs |
Forward Delay |
Duration of Listening and learning states |
Secs |
Max Age |
Time BPDU Stored |
Secs |
Hello TimeControl is the time interval for sending configuration BPDUs, 802.1D specifies that the default value for this interval is 2 seconds, note that this value only controls the root bridge send configuration Bpdu period, and other bridges only do forward BPDUs received from the root bridge. That is, because a network failure does not receive bpdus within 2-20, the non-root bridge stops periodically sending BPDUs during this time (if the BPDUs are not received for more than 20 seconds, the bridge-saved BPDUs are invalidated directly, and the new root bridge is re-selected). All bridges use their locally configured Hello time as the TCN transmission cycle, which will be later "topology change Notification BPDUs"This section discusses. Forward Delayis the network Bridge monitoring and the delay time period in the learning State. The default value for this 15-second is originally assumed to be the network maximum hop digit 7, the maximum loss of BPDUs is 3, and a 2-second cycle of Hello time (for a detailed introduction to how to calculate forward delay in the seventh chapter of the "Tuning Forward Delay"section). As described in the section topology change Notification bpdus ",Forward delay also controls the aging time period of the bridge forwarding post when the network topology changes. Max Age is the time period in which the Bridge saves the BPDUs before discarding them, recalling that each port that was mentioned earlier saved one of the best bpdus it received. Once the bridge begins to receive a continuous BPDU every 2 seconds, the receiving bridge continues to hold a copy of the BPDU. However, if the optimal BPDU send fails, the other bridge will take over the job in the future. For example, suppose that the third link in Figure 6-12 uses a hub connection and the transmit and receive function of the CAT-B:PORT-1/2 fails, and Cat-c still receives the Ethernet link information from the hub without immediately getting the stale information, the only thing that knows is that the BPDUs are not continuously sent. After this 20-second (Max-age) error occurs, CAT-C will age out of expired BPDU information: Cat-b has the optimal designated port for the third link. Causes the CAT-C:PORT-1/2 to transition to the listening state and attempts to become the specified port, because CAT-C:PORT-1/2 is the port that the link is currently most effective in accessing to the root bridge, and eventually translates to forwarding mode. In fact, the port has a state switch after the Cat-b port PORT-1/2 error occurs 50 seconds (10 seconds max age + 15 seconds listening State time +15 learning State time). in some cases, the bridge detects changes in the topology on a direct-attached link and does not need to wait for the Max age time period to immediately transition to the listening state. For example, consider the scenario in Figure 6-13.
Figure 6-13. Failure of a Link Directly Connected to the Root Port of Cat-c
in this example, the CAT-C:PORT-1/1 error, because it is the root port link error, all no need to wait 20 seconds has caused the previous information aging, instead of CAT-C:PORT-1/2 immediately into the learning mode and try to become the new root port. This reduces the STP convergence time from 50 seconds to 30 seconds (15 seconds of listening state time + 15 seconds of learning state time). Tips:STP default STP convergence time is 30 seconds to 50 seconds. The seventh chapter "Fast STP Convergence" discusses how to improve this convergence problem.
the use of STP timer should pay attention to two points. 1th, change the timer default time value before you have to deliberate, in this chapter seventh has more detailed discussion. 2nd, even if you are confident about the timer adjustment, you should only modify the timer on the root bridge. As you will see in the following section, the BPDU contains three timer fields, which are passed through three fields to pass the timer value to the rest of the networkAllNetwork Bridge. Consider the other case, each bridge is using local configuration, some bridges in their own way in the other bridges have not left the listening state is into the forwarding state, this chaotic operation of the obvious mess up the network. However, by providing a timer field in the BPDU, the root bridge can tell the configuration of the time parameter to the network bridge across the net. Tips:You can only modify the timer value of the root bridge, and modifying the value on the non-root bridge has no effect. However, do not forget to update the Network Bridge timer values that may later become the root bridge.
The classic "Cisco Lan Switching" chapter sixth (vii): three STP Timers
