A student has built a network as shown in the below diagram and found the partial configuration on switch S$1.$

Use the knowledge you have learned from this subject, and answer the below question briefly in the text box.

vlan$\_$native VLAN, SVI.png

After assigning the IP address and enabling the interface VLAN $99,$ the student tried to ping $172\mathrm{.}17\mathrm{.}99\mathrm{.}2$ from PC$1.$ However, the ping was not successful, although PC$1$ could ping PC$2$ successfully.

What could be the reason$($s$)?$ Include the commands that you would use for troubleshooting