What EtherChannel does and what prevents it from forming
EtherChannel bundles 2 to 8 physical links between the same two switches into a single logical port-channel interface. STP treats the entire bundle as one link, so none of the member ports are blocked. Traffic distributes across member links using a hash algorithm, and if one member link fails, the others continue forwarding without STP needing to reconverge.
All member ports must have identical configuration or the channel will not form. Speed, duplex, VLAN configuration (same allowed VLANs on trunks, same access VLAN on access ports), trunk or access mode, and native VLAN must all match. A single mismatch prevents the entire bundle from forming. When a channel fails to form, this is the first thing to check.
LACP, PAgP, and static modes
LACP (Link Aggregation Control Protocol) is the IEEE 802.3ad open standard. It has two modes: Active, which sends LACP negotiation frames and initiates the channel, and Passive, which responds to LACP frames but never initiates. Active-Active forms a channel. Active-Passive forms a channel. Passive-Passive never forms because neither side initiates. Remember: at least one side must be Active.
PAgP (Port Aggregation Protocol) is Cisco-proprietary. Desirable mode initiates negotiation. Auto mode responds but does not initiate. Desirable-Desirable forms. Desirable-Auto forms. Auto-Auto does not form because neither side initiates. The pattern is identical to LACP: one side must be the initiator.
Static mode, configured as on, forces the channel with no negotiation protocol. Both sides must be set to on. Mixing on with LACP or PAgP modes does not form a channel because the static side is not speaking any negotiation protocol.
Traffic distributes across member links per-flow, not per-packet. All packets in the same flow use the same physical link based on a hash of source and destination MAC or IP. This guarantees in-order delivery within each flow and is why a single large file transfer may not use all member links.
How to choose the correct answer
LACP mode combinations: Active-Active = forms. Active-Passive = forms. Passive-Passive = does NOT form.
PAgP mode combinations: Desirable-Desirable = forms. Desirable-Auto = forms. Auto-Auto = does NOT form.
Static: On-On = forms. On with any LACP or PAgP mode = does NOT form.
Cannot mix LACP and PAgP on the same channel. Both sides must use the same negotiation protocol.
Channel not forming troubleshooting: check that one side is Active (LACP) or Desirable (PAgP), then verify all member port configurations match.