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The 8021w standard introduced a change with BPDUs Some additional flags were added to the BPDUs, so that switches could share information about the role of the port the BPDU is exiting or leaving This can help a neighboring switch converge faster when changes occur in the network In 8021d, if a switch didn t see a root BPDU within the maximum age time (20 seconds), STP would run, a new root switch would be elected, and a new loopfree topology would be created This is a time-consuming process With 8021w, if a BPDU is not received in three expected hello periods (6 seconds), STP information
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14: Switches and Redundancy
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can be aged out instantly and the switch considers that its neighbor is lost and actions should be taken This is different from 8021d, where the switch had to miss the BPDUs from the root here, if the switch misses three consecutive hellos from a neighbor, actions are immediately taken
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The 8021w standard includes new convergence features that are very similar to Cisco s proprietary UplinkFast and BackboneFast features The first feature, which is similar to Cisco s BackboneFast feature, allows a switch to accept inferior BPDUs Look at Figure 14-11 to understand the inferior BPDU feature In this example, the root bridge is Switch-A Both of the ports on Switch-B and Switch-C directly connected to the root are root ports For the segment between Switch-B and Switch-C, Switch-B provides the designated port and Switch-C provides a backup port (a secondary way of reaching the root for the segment) Switch-B also knows that its designated port is an alternative port (a secondary way for the switch to reach the root), via Switch-C from Switch-C s BPDUs Following the example in Figure 14-11, the link between the root and Switch-B fails Switch-B can detect this by either missing three hellos from the root port or detecting a physical layer failure If you were running 8021d, Switch-B would see an inferior root BPDU (worse cost value) coming via Switch-C, and therefore all ports would have to go through blocking, listening, and learning states, which would take 50 seconds, by default, to converge With the inferior BPDU feature, assuming that Switch-B knows that Switch-C has an alternate port for their directly connected segment, then Switch-B can notify Switch-C to take its alternate port and change it to a designated port, and Switch-B will change its designated port to a root port This process takes only a few seconds, if even that The second convergence feature introduced in 8021w is rapid transition Rapid transition includes two new components: edge ports and link types An edge port is
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FIGURE 14-11 ROOT Switch-A
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Accepting inferior BPDUs
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Switch-C
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Switch-B
Rapid Spanning Tree Protocol
a port connected to a non layer 2 device, such as a PC, server, or router RSTP with rapid transition of edge ports to a forwarding state is the same as Cisco s proprietary PortFast Changes in the state of these ports do not affect RSTP to cause a recalculation, and changes in other port types will keep these ports in a forwarding state Rapid transition can take place in RSTP only for edge ports and links that are point-to-point The link type is automatically determined in terms of the duplexing of the connection Switches make the assumption that if the port is configured for full-duplex between the two switches, the port can rapidly transition to a different state without having to wait for any timers to expire If they are half-duplex, this feature won t work by default, but you can manually enable it for point-to-point half-duplex switch links Let s take a look at an example of rapid transition of point-to-point links by using the topology in Figure 14-12 The topology in Figure 14-12 is the same as that shown in Figure 14-11 In this example, however, the link between Switch-A (the root) and Switch-C fails When this happens, Switch-C can no longer reach Switch-A on its root port However, looking at the BPDUs it has been receiving from Switch-A and Switch-B, Switch-C knows that the root is reachable via Switch-B and that Switch-B provides the designated port (which is in a forwarding state) for the segment between Switch-B and Switch-C Switch-C, knowing this, changes the state of the backup port to a root port and places it immediately into a forwarding state, notifying Switch-B of the change This update typically takes less than a second, assuming that the failure of the segment between the root and Switch-C is a physical link failure, instead of three missed consecutive hello BPDUs
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