Cat-D> (enable) set spantree uplinkfast enable VLANs 1-1005 bridge priority set to 49152 The port cost and portvlancost of all ports set to above 3000 Station update rate set to 15 packets/100ms. uplinkfast all-protocols field set to off uplinkfast enabled for bridge
First, the Bridge Priority is modified to an unusually high value of 49,152. This causes the current switch to effectively take itself out of the election to become the Root Bridge. Second, it adds 3000 to the cost of all links. This is done to discourage other switches from using the current switch as a transit switch to the Root Bridge. Notice that neither of these actions limits STP failover in your network. The Bridge Priority modification only discourages other switches from electing this switch as the Root Bridge. If the other switches fail, this switch happily becomes the Root Bridge. Also, the increase to Path Cost only discourages other switches from using the current switch as a transit path to the Root Bridge. However, if no alternate paths are available, the current switch gleefully transfers traffic to and from the Root Bridge.
Notice the third line in the output in Example 7-16 (in bold). This is evidence of a subtle trick that is the crux of what UplinkFast is all about. It should probably be fairly obvious by now that a failure on Cat-D:Port-1/1 forces Cat-D to take all MAC addresses associated with Port 1/1 in the Bridging Table and points them to Port 1/2. However, a more subtle process must take place to convert the bridging tables in other switches. Why is this extra step necessary? Figure 7-23 shows the network with the left-hand link broken.
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