Understanding Linux Network Internals

Convergence Time

We have seen how the STP dynamically updates the topology of the tree based on configuration changes and link or bridge failures. Let's see now how much time STP needs to detect common failures and react accordingly.

When a configuration update takes place on a complex scenario, the network may require minutes before it converges and stabilizes.^[*] During that time the topology is still loop free, but it may not be able to carry traffic properly (because the topology is still changing while the traffic is in transit). In those setups, it is not possible to predict exactly how the topology evolves toward a new stable tree, because the timing of BPDU receptions and transmissions depends on several factors, such as how loaded the bridges are at that moment.

However, no matter how well you configure the bridges, there are minimum latencies that cannot be eliminated or reduced. For example:

• When a port changes state, moving, for example, from blocking to forwarding to replace a failing bridge port, the transition to forwarding is not immediate, but takes twice the time of the Forward Delay timer (i.e., 30 seconds by default), as shown in Figure 15-6. The port cannot forward any data traffic during this time.

• Root and nondesignated ports (i.e., the ones that receive BPDUs) realize that they have lost connection to their designated bridge (and therefore to the entire tree except for the portions below the bridge's designated ports^[*]) only when their Message Age timer expires. ...