What must be true about the EtherChannel channel-group number on neighboring switches?

For EtherChannel configuration to function correctly between neighboring switches, the channel-group number does not need to match. Each switch can independently assign a different channel-group number when configuring EtherChannel, as long as the physical interfaces and parameters (such as protocol and speed) are compatible on both ends.

The channel-group number is essentially a local identifier used by each switch to manage its own aggregate link formation. Therefore, it's acceptable for one switch to use a certain channel-group number while the neighboring switch uses a different number for the same EtherChannel. This design allows for flexibility in configuring EtherChannels across switches in a network environment.

In certain setups, it's advantageous for clarity and ease of management to use the same channel-group number on neighboring switches, but this is not a requirement for the EtherChannel to be operational.