As shown in Figure 13.5, each of the customer bridges is aware of the two VLANs
and keeps the MAC addresses in the two VLAN separate in its filtering database. Each
learns that dotted A is on one port and solid A is on another port.
Some of the provider bridges, particularly bridge R, have a problem, however. Once
the customer??™s frames enter the provider??™s network, they have an outer S-tag applied.
The provider bridges forward frames based on the S-VID and the MAC addresses in
the frame. They do not look inside the frame at the C-tags. (See ???Proper Layering???
next) Bridge R sees a frame with source MAC address A on the gray S-VLAN coming
from both directions and cannot tell in which direction to send frames from D bound
for (solid) A.
Note that there is no confusion in the provider??™s network among different customers.
MAC addresses in the gray S-VLAN are kept completely separate from any other
customers??™ MAC addresses. The problem is strictly one of duplicated MAC addresses
within a single S-VLAN.
Fortunately, this situation is relatively easy for the customer to avoid. One way is
for the customer to configure VRRP so the two routers do not share a common MAC
address.
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