Channel-group modes

[Brandon Koehler]

I inherited a network with a 3750 stack that functions as the core of the network. (Not really core switches, I know) I am cleaning up a bunch of half done stuff and noticed that all 12 of the port-channel groups are in 'on' mode. I've read that this isn't really best practice and when you create these things with Cisco gear at both ends (as this is), you want to use 'desirable' mode.

Long story short, would it be worth it to tear down all the port-channels and rebuild them in 'desirable' mode? Everything is working perfectly now.

Thanks.

[rizwanr74]

Well,  'desirable' is DTP mode.

When it comes to port-channeling there are three modes, "Active", "Passive" and "on".

"on" is just being on, no negotiation.

Hope that answers your question.

thanks

[Ryan Wolfe · Accepted solution]

Brandon,

Unfortunately, the above poster had some wrong information. While "desirable" is a DTP negotiation mode, it is also a ether channel mode. There are two protocols for ether channel negotiation. PAgP is the Cisco proprietary protocol that employs desirable and auto, and LACP is the open standard that uses active and passive. As far as negotiation is concerned, active and desirable both actively try to become a trunk, while auto and passive only become one if told. On and off manually assign the mode and don't even participate in the negotiation process.

It is probably Cisco best practice to use desirable as the mode for your port channels because it is Cisco's protocol, but manually assigning it is just as good unless you replace a switch and don't set one side to on. Then the port channel would fail on that port until it was fixed.

Personally, I don't see it making much of a difference, but you could change it to desirable one half of the channel at a time, and no down time would occur.

Take a look at this: http://www.cisco.com/en/US/products/hw/switches/ps5023/products_configuration_example09186a00806cb982.shtml#lacp

HTH,

Ryan

Sent from Cisco Technical Support iPad App

[glen.grant]

  Between cisco devices i have always used the "desirable non-silent"  option . Has always worked fine and we never have any issues .  Prevents any misconfiguration problem that hardcoded channels can cause.  If they have been working ok with on mode you won't see any different.  It will just prevent any misconfigurations in the future .

[Brandon Koehler]

The link cleared everything up. These ports are split between two different switches in the stack, and apparently you can't use 'desirable' between two different switches.

I'll leave them as-is. Thanks.

[wpalumbo06]

Brandon,

As stated above, cross stack port channels must use mode on.  However, I would strongly consider discontinuing the use of mode on where possible on new installs.  I have always used mode on and that has bit me twice (very badly) in recent months.  The examples listed in this thread are correct and I had a very bad experience with a 4506 that lost it's config after a power failure - and no, I wasn't the one who forgot to save the config:-)   This 4506 was port channeled to a 6509 with mode on and this created bridging loop that caused massive problems for the users in the building (not just the ones connected to the switch with no configuration).  Bottom line is I have moved away from mode on in favor of using LACP as a standard as that's what the Nexus switches support.

Bill

[John Blakley]

Brandon,

As others have said, I wouldn't personally change it if it's been working unless you're going to a standard. I don't believe you can change one side without changing the other because one of the stipulations of using the ON mode is that both sides need to be in the ON mode before coming up. If you decide to change it, I would create an outage window before doing so. You're not going to notice any difference with speeds, configuration of the individual ports, or anything else of that nature. This mode still falls under the same rules as lacp and pagp modes in that the individual ports all have to be configured in the same way.

HTH,

John