Correct Answer: D
EtherChannel can be established using either the Port Aggregation Protocol (PAgP), which is Cisco proprietary, or the Link Aggregation Control Protocol (LACP), which is part of an IEEE standard. Both protocols facilitate the automatic creation of EtherChannel links.
In the scenario provided, the EtherChannel issue between SW2 and SW3 can be resolved by configuring both interfaces to use LACP, which is indicated by the 'mode active' setting. This settingallows the interfaces to actively negotiate an EtherChannel link with connected devices that are also set to either active or passive mode for LACP.
The 'mode active' configuration ensures that both switches will send LACP packets to each other, attempting to establish the EtherChannel link. If both sides are set to 'mode active', they will form an EtherChannel if they are able to negotiate successfully.
References:
* Implementing and Operating Cisco Service Provider Network Core Technologies (SPCOR) source book
* Cisco documentation on EtherChannel configuration and troubleshooting