How many Spanning Tree domains are shown in Figure 7-31? Assume that all of the switches are using ISL trunks and PVST Spanning Tree.
Although the same numbers are used for all of the VLANs, the routers break the network into four Layer-2 pockets (Cat-1 through Cat-3, Cat-4, Cat-5 through Cat-6, and Cat-7 through Cat-10). The VLANs in each Layer-2 pocket then form a separate STP domain.
One of the tricks in this layout is to notice that Cat-5 and Cat-6 form a single Layer-2 domain containing two, not three VLANs. Because of the backdoor links between the two switches, the routers do not break this into separate Layer-2 pockets.