In Figure 13.12, this customer??™s EVC is assumed to be confined to a single spanning
tree rooted at bridge W. Only the physical links shown as solid lines are used to carry
the EVC. Compared to Figure 13.11, traffic between one pair of customer sites, C1-C3,
takes a longer path through the network. However, only this one pair suffers; the other
five possible pairings take optimal paths. The amount of bandwidth required by the
customer on any given link can reasonably be set by the provider, and debugging problems
is simpler. More importantly, if you assume that the network shown is a provider
Figure 13.11 Direct routing of an EVC
C 1
C 3
C 2
C 4
S
T
U
V
W
X
Y
Z
Ethernet Bridging 401
bridge (802.1ad) network, only bridge W in Figure 13.12 learns this customer??™s MAC
addresses. All the other bridges, S-V and X-Z, have only two ports each that carry this
customer??™s data, and hence they do not need to learn any customer MAC addresses.
By contrast, in Figure 13.11, every bridge except bridge V has at least three ports carrying
this customer??™s data, and hence all but that bridge must learn at least some of
this customer??™s MAC addresses.
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