Final answer:
The three pairs of trunking modes that successfully establish a trunk link between two Cisco switches are Dynamic Desirable - Dynamic Desirable, Dynamic Desirable - Dynamic Auto, and Dynamic Desirable - Trunk. Both Dynamic Auto - Dynamic Auto and Access - Dynamic Auto pairs are incapable of forming a trunk on their own.
Step-by-step explanation:
The question relates to the establishment of a trunk link between two Cisco switches using different switchport trunking modes. A trunk link is a link designed to carry multiple VLANs through a single network link through the use of a trunking protocol like IEEE 802.1Q. In Cisco switches, the switchport mode settings define how the switchport will behave with regard to trunking. Here are the three pairs of trunking modes that will establish a functional trunk link between two Cisco switches:
- Dynamic Desirable - Dynamic Desirable: Both switches actively attempt to negotiate a trunk link between them.
- Dynamic Desirable - Dynamic Auto: One switch (dynamic desirable) actively attempts to negotiate a trunk, while the other switch (dynamic auto) is willing to become a trunk link if the other side initiates the negotiation.
- Dynamic Desirable - Trunk: One switch is actively trying to initiate trunking (dynamic desirable), and the other side is set to trunk mode, which means it will stay as a trunk link irrespective of the other side's actions.
The access - dynamic auto and dynamic auto - dynamic auto pairings will not form a trunk because in both cases none of the switches is actively trying to initiate the trunking. The access mode forces the port to become a part of a particular VLAN and restricts its ability to carry multiple VLANs, while dynamic auto is a passive mode that will switch to trunk only if the other side initiates the process. So, they are not valid trunking pairs to form a trunk link.