Whenever a new EVC is added, it can be assigned to a VLAN carried on
a spanning tree appropriate for the customer??™s needs. Only the bridges on which the
Figure 13.12 Spanning tree routing of an EVC
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C 3
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C 4
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402 Chapter 13
customer UNI ports reside need be configured; all other ports are automatically activated
by MVRP. Unused VLANs not assigned to customers cost absolutely nothing for
the network control protocols to maintain.
Reliable Roots Any properly designed network contains redundant paths so a single
failure cannot split the network. (These redundant paths are, of course, the reason that
routing protocols and/or spanning tree protocols are required.) The convergence time
problem caused by the ???chatty timer,??? which results in convergence times greater than
1 sec, occurs only when a network port is separated from the root of a spanning tree.
Thus, the failure of the root bridge of a spanning tree can trigger a network disconnect
(for the users of that spanning tree) that is greater than 1 sec. Single link failures
should not cause this long outage.
A network can be constructed to avoid this type of slow convergence, in the face of a
single failure, by making, at least, those bridges that serve as spanning tree roots extra
reliable.
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