Interconnecting Clouds of Bridges An EVC with three ports, as shown in Figure 13.5,
looks to the customer??™s spanning tree algorithm just like the large diameter coax cable
in Figure 13.2. There are some things that a customer must be very careful of, however,
if he or she sends BPDUs over a provider??™s network.
First of all, the provider??™s network components are not, of course, perfectly reliable.
When a failure, an addition to the network, or a significant configuration change takes
place, the flow of customer data may be interrupted until the provider??™s spanning tree
algorithm can correct the situation. If both the provider??™s and the customer??™s spanning
tree algorithms are using the same default timers for the transmission and timeout
intervals, then there is a very real chance that both networks will detect an error when
a provider change occurs. When the provider has restored connectivity, the customer
may experience a temporary data loop.
The customer can avoid this problem simply by increasing the value of the port hello
time on those ports that face the provider. The default is 2 sec. You can ask the provider
what time is appropriate for a customer network that is transmitting BPDUs over the
provider network, but a value three times larger than the provider??™s bridge hello time
is appropriate.
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