When that limitation is exceeded, it can be split
into multiple Q-in-Q clouds interconnected by a small MAC-in-MAC backbone, and so
on, as hinted at in Figure 13.1.
An IEEE 802.1ah MAC-in-MAC backbone network can also connect directly to a
customer; an intervening 802.1ad Q-in-Q network is not required. Using a MAC-in-
MAC network to connect directly to the UNIs has the obvious advantage of eliminating
the Q-in-Q technology from the picture, thus simplifying management and training. It
scales better than Q-in-Q alone, because more than 4094 EVCs are available. However,
the MAC address of each I-component in the network (see ???The I-Component???) is carried
across the backbone. As the backbone network extends out to the edge, it gets
bigger and bigger, and the number of I-components increases. Eventually, the network
gets larger than the spanning tree protocols can handle, even after being extended by
the various techniques given in this chapter. As these issues approach, Q-in-Q networks
can be added to separate the backbone from the customers, and the network grows
toward the complete picture shown in Figure 13.1.
ISP Access Services
As shown in Figure 13.
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