Link Aggregation - LACP

Link Aggregation - LACP

I feel there is a lot of confusion regarding Link Aggregation.. features like system-priority.. port-priority... non-silent.. silent.. active, passive, desirable, auto, on.. ah! Unfortunately this is all part of the package.. we gotta know it all..

There are three flavors of Etherchannel: LACP, PAgP, and "ON" - each of these "modes" offer distinct advantages (and disadvantages) to their competitors. Most commonly I think you will see "LACP" or Link Aggregation Control Protocol used in the majority of situations. LACP is a standards based solution to link aggregation, it can negotiate an EtherChannel with any vendor device (HP, Dell, Juniper, whatever you want) without any hastle. The caveat with LACP is some of the more detailed features such a the system-priority and port-priority settings. Technically LACP can bundle 16 ports together in one logical EtherChannel port.. however the caveat here is that 8 of these links will be in a standby mode and 8 will be active. The priority for which are active or standby is in the port-priority setting, configured on a port-by-port basis. If you don't care which ports are active or standby, then you just leave it default at 32768 priority.. in which case the lowest numbered ports will have priority. (Lower is higher priority in LACP - remember this!) In addition to this priority setting, there is a system-priority setting which is configured globally on a switch. The lower the system-priority on a switch, the more likely it is to be the "master" in an etherchannel negotiation with another switch. One switch needs to be designated the leader or master, and this is how that decision is made in the LACP protocol. The option to use 16 redundant links in a single etherchannel is slick.. but do remember you are capped at 8 active links at any once time.

The last important piece of information are the LACP "modes"; active and passive. Active mode is a mode which ACTIVELY tries to negotiate with the neighbor switch.. while passive sits back and waits to be negotiated with. Needless to say (I hope) passive ----> passive will NOT work in forming an Etherchannel link. Passive ---- > Active will work however.. Two passive links will send no LACP negotiation packets whatsoever, and permanently remain unchanneled.

Lastly, an interesting factoid I discovered through experimentation was the difference between LACP Slow and Fast PDU packets. If you do a "show etherchannel port" you may see Slow PDU or Fast PDU's beside your link status. I was worried at first when I saw the slow PDU status on my links - however this is completely normal. Slow PDU's are used when the Etherchannel is established and stable (every 30 seconds), while Fast PDU's are used when the Etherchannel is first forming (every 1 second!)

Bye for now...

Kyle