Check the Allowed VLAN List on Both Ends of a Trunk
The next item, and the one that follows, both occur when an engineer makes some poor configuration choices on a VLAN trunk. In real life, you should instead just configure the trunk correctly, as outlined in Chapter 9’s section “VLAN Trunking Configuration” and the section that follows it, “Controlling Which VLANs Can Be Supported on a Trunk.” But for the exams, you should be ready to notice a couple of oddities that happen with some unfortunate configuration choices on trunks.
First, it is possible to configure a different allowed VLAN list on the opposite ends of a VLAN trunk. When mismatched, the trunk cannot pass traffic for that VLAN.
Figure 10-6 shows an example. Both switches have defined ...
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