200-301: Cisco Certified Network AssociateDemo

To meet the requirements, the inter-switch link must remain a trunk, continue carrying the existing VLANs, add VLAN 20, and use IEEE 802.1Q rather than Cisco ISL because 802.1Q is the IEEE standard trunking method. The switchport trunk allowed vlan add 20 command extends the existing allowed VLAN list without removing the VLANs already permitted, and switchport trunk encapsulation dot1q selects the IEEE-standard encapsulation. That is why the configuration in choice D is correct. Cisco documentation identifies ISL and IEEE 802.1Q as the two trunk encapsulations and shows that 802.1Q is the IEEE standard, and Cisco command references document switchport trunk allowed vlan add ... for adding VLANs to an existing trunk allowed list.