IEEE 802.1aq Shortest Path Bridging, by replacing the distance vector algorithms
with link state algorithms, will fix this problem in a general way. Whether SPB will give
convergence times as fast as the proprietary ring algorithms remains to be seen.
Paired Virtual Bridges Several vendors offer another alternative to spanning tree, or
at least to minimizing the size of the network protected by spanning trees??”the virtual
bridge pair. A network employing virtual bridge pairs is shown in Figure 13.14. This
technique allows a provider to build an extremely large network using a minimum of
spanning tree techniques.
In Figure 13.14, you have a core network, simplified to only four bridges A??“D. This
network is likely a mesh network, in order to carry such a massive load of data, but it
could be another topology such as a ring if sufficient bandwidth is available. Each bridge
in the core is a completely separate entity as far as the control plane is concerned.
At the distribution layer, Figure 13.14 shows a number of paired virtual bridges E
through K. Each pair of bridges looks, to the rest of the network, like a single bridge.
Presumably, the provider would take the precaution of using separate power sources
for each bridge in the pair.
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