Etherchannel - PAgP and LACP?

[miketta89]

I'm studying for the CCNP BCMSN exam and have a question regarding the use of Etherchannel.

I understand what PAgP and LACP do but why would you use them to setup an etherchannel link?

I mean you still have to configure each side to use PAgP or LACP so why not just turn the ports to "On" and be done without adding another protocol into the mix?

The only reason I can think of to use LACP is if you want to keep standby links available in case one of the active 8 links goes down. fair enough, but PAgP?

If you have 8 ports in a port channel from switch A to switch B what benefit would you get out of using PAgP or LACP over just turning the mode to On?

How do you guys setup Etherchannel links in a real environment?

Thanks!

[glen.grant]

If using all cisco devices we use pagp just because it has been around the longest and is very stable to setup the channels . Negotiating the channels lets the switch look at different parameters and makes sure they are all the same on each end so no problems occur , if they are not then the switch will not form the channel .LACP is the IEEE standard for etherchannel . A major warning out of a cisco doc.

Serious traffic problems can result from mixing manual mode with PAgP or LACP modes, or with a port with no EtherChannel configured. For example, if a port configured in on mode is connected to another port configured in desirable mode, or to a port not configured for EtherChannel, a bridge loop is created and a broadcast storm can occur. If one end uses the on mode, the other end must also.