For traffic from a VLAN to be transmitted across a trunk, it must be allowed on the trunk. To do so, use the switchport trunk allowed vlan vlan-id command.
In Figure 1, VLAN 20 (Student) and PC5 have been added to the network. The documentation has been updated to show that the VLANs allowed on the trunk are 10, 20, and 99. In this scenario, PC5 cannot connect to the student email server.
Check the trunk ports on switch S1 using the show interfaces trunk command as shown in Figure 2. The command reveals that the interface F0/3 on switch S3 is correctly configured to allow VLANs 10, 20, and 99. An examination of the F0/3 interface on switch S1 reveals that interfaces F0/1 and F0/3 only allow VLANs 10 and 99. Someone updated the documentation but forgot to reconfigure the ports on the S1 switch.
Reconfigure F0/1 and F0/3 on switch S1 using the switchport trunk allowed vlan 10,20,99 command as shown in Figure 3. The output shows that VLANs 10, 20, and 99 are now added to the F0/1 and F0/3 ports on switch S1. The show interfaces trunk command is an excellent tool for revealing common trunking problems. PC5 has regained connectivity to the student email server found at IP address 172.17.20.10.