Thus, you can see by this comparison that having a
single spanning tree for a given customer can be, on balance, an improvement over the
???perfect??? routing of data.
Multiple Paths for Customers??™ Data Clearly, as shown in Figure 13.12, it is not desirable
for a provider network to leave seven links unused. This is one reason why the Multiple
Spanning Tree Protocol (MSTP) was developed. While any given VLAN, and therefore
any given customer, is confined to a single spanning tree, up to 64 spanning tree instances
may be constructed in a single provider network, which, in practice, is far more
than are necessary to get excellent usage of all the network??™s links. In Figure 13.12,
for example, adding a second tree rooted at bridge V would ensure that all of the links
except T-Y is in use. Even the largest real networks seldom need more than four to six
instances of the spanning tree.
To avoid needless reconfiguration of the bridges each time a new EVC is added to
the network, best practice is to create all of the spanning trees at the start and assign
each of the 4094 possible VLANs to a spanning tree, even though only a few may initially
be used.
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