An interesting situation for the customer who runs BPDUs over the provider network
is shown in Figure 13.6. In this figure, the customer is using the provider network
as a four-port E-LAN EVC connecting four ports in the customer??™s two clouds. The purpose
of the EVC is to provide redundant connectivity for each cloud, so the two clouds
can have maximum connectivity in the face of failures. Presumably, each of the eight
ports shown as small black circles in Figure 13.6 are in eight different bridges in the
provider??™s and the customer??™s network.
Figure 13.6 Customer spanning tree carried over the provider network
y
y
1
y
1
y
1
y
1
Provider cloud
(looks like big coax)
Customer cloud 1
(Root here) Customer cloud 2
One of
these two
ports must
block.
Some interior
port, perhaps
this one,
must block!
R
X
X
390 Chapter 13
The root bridge of the customer??™s spanning tree must be somewhere; we assume it
is in the left cloud in Figure 13.6. One of the ports from this cloud must be blocked
in order to prevent frames from looping through the provider network in and out of
the left cloud. What is interesting in Figure 13.
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