Mirza:

Etherchannel is used to bundle multiple physical inter-switch ethernet links together so that the switches see them as only ONE logical link, instead of 3, 4 or 5 (up to 8). This will allow you to use the bandwidth of those redundant links without having them blocked by STP.

Example:

Switch A and switch B are connected to each other via one dot1q trunk. If you add another dot1q trunk, STP will block that trunk from forwarding user-traffic because it does not allow redundant links that carry the same vlan traffic to exist. It may cause a loop. If you configure ehterchannel on both switches, however, the switches' STP process will not block any additional trunks because they will all be placed in one etherchannel channel group and be seen as only one link.

Anyway, for LACP, which is an open standard, the choices you have when you configure ports to negotiate an etherchannel are Passive and Active.

If two neighboring ports are configured for Active, an etherchannel will be established.

If one side is set for Active and the other for Passive, an etherchannel will also be formed.

If both sides are set for Passive, an etherchannel will NOT be formed, since each side will be waiting for the other to begin negotiating.

Now for PaGP, which is Cisco's etherchannel protocol, the choices are Desirable and Auto.

If two neighboring ports are configured for Desirable, an etherchannel will be established.

If one side is set for Desirable and the other for Auto, an etherchannel will also be formed.

If both sides are set for Auto, an etherchannel will NOT be formed, since each side will be waiting for the other to begin negotiating.

As for configuring them for "on," there will be NO negotiating and both sides MUST be configured for "on" for the etherchannel to form.

HTH

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Victor