EtherChannel - LACP: Confused between "active" mode and the lower system-priority

[danielseijas3010]

Hi Everyone,

I am studying for my CCNP switching exam, currently reading the chapter about EtherChannel. My doubt is with regards LACP, lets start with some background context just to make sure my theory is accurate:

A.- When a port is configured in a group-channel as "Active" LACP mode, it means that the port will actively send LACP negotiation packets to the far end device waiting for a response in order to bundle the link into the channel. If the port is configured as "Passive", that means that the port will agree to negotiate but the far end is supposed to start sending the LACP packets.

B.- LACP introduces the concept of "system-priority" and "port-priority" (very similar to their STP counterparts). This feature allows you to configure more links than allowed into the EtherChannel bundle, and then the switch with the lowest system-priority will decide which ones of these ports will be bundled into the channel and which ones will be in standby in case one of the latter goes down. These decisions will be based on the "port-priority".

If my concepts are wrong, please let me know. Now my questions are related to 4 particular scenarios:

SCENARIO 1: Let's assume I have two LACP-capable switches connected to one another with 16 ethernet cables. All of them identically configured (on both switches) and ALL OF THEM IN "ACTIVE" LACP MODE. At this stage, one of the switches will be the decision maker and will decide which 8 of the available ports (the ones with lowest port priority) will be bundled. What happens with the non-decision-maker switch? Will its ports just start sending LACP packets in a "blindfolded" manner until the decision makers shut 8 of their ports down?

SCENARIO 2: Same scenario but now one of the switches is configured with all of its 16 LACP ports as "ON" mode (the other switch still has all of its ports in Active mode). How will the "active" switch know if it is the decision maker or not?, given that its far end partner is not sending LACP packets at all.

SCENARIO 3: What happens if both switches are have configured all of these ports as LACP "ON" mode. We know that only 8 links are possible in an EtherChannel, but how will they negotiate which ones will be selected? both in ON mode mean that they are not negotiating or sending any LACP packets.

SCENARIO 4: Same question as SCENARIO 3, but if both switches have configured all of these ports as LACP "passive" mode. How will they decide which ports to bundle if none of them have started sending packets?.

I hope my questions make sense. Thank you so much!

[danielseijas3010]

I think I am getting some answers while I am reading other posts:

SCENARIO 2: the EtherChannel will never be formed

SCENARIO 4: The EtherChannel will never be formed (I suspected it :))

I hope you guys can answer SCENARIO 1 (which is mainly a confirmation) and SCENARIO 3!!!

Thanks again

[Peter Paluch · Accepted solution]

Hi Daniel,

A) Correct.

B) Correct.

Scenario 1: The non-decision-maker switch will truly start sending LACPDUs "blindfolded". The fact that it is a non-decision-maker will actually be determined only by both switches sending LACPDUs to each other and comparing their LACP System IDs. If either of these switches was silent at the beginning, the other switch would be unable to learn its LACP System ID and determine who is going to be the decision-maker. Also please note that the 8 hot-standby ports will not be shutdown - they will only be "hibernated" but not entirely brought down.

Scenario 2: There is no such thing as an LACP ON mode. You can either have mode active or mode passive with LACP. The mode on is a static mode without any signalling protocol, neither LACP nor PAgP. A combination of LACP (any mode) on one side and on on the other side will not result into a working EtherChannel, as the LACP side will be waiting for LACPDUs from the other switch that will never come.

Scenario 3: Again, there is no LACP ON mode. With mode on, there is no negotiation, and you can bundle at most 8 ports in a static on mode - they won't negotiate, they will just be bundled and used right away.

Scenario 4: If both sides are set to mode passive, neither of them will start sending LACPDUs. As a result, the EtherChannel will not be negotiated and will not come up. At least one switch must be configured with mode active in order for an LACP EtherChannel to come up.

Best regards,
Peter

[danielseijas3010]

Thanks Peter! Everything is almost clear now. One more thing about Scenario #1: Is hibernating enough to recognize a link 'DOWN" at the far end?, and if the answer is NO, how will the far end switch know what ports not to forward packets from.

[Peter Paluch]

Hi Daniel,

The links will not be down - they will simply be negotiated as standby links by LACP. The switch is intelligent enough to know that if the links are considered standby, they should not be used to carry traffic.

Best regards,
Peter