EtherChannel Checks

[PrimeYeti]

Does an EtherChannel perform checks on a new interface or an opposite switch even if it is enabled manually? E.g. not using LACP or PaGP. I would assume that it does but I couldn't find a clear answer.

Further to this, are all the checks performed on a new port being added (e.g. Speed, Duplex, Access/Trunk, VLANs, STP) also run between switches to ensure interfaces at each end of the channel match?

[David Ruess]

Hello,

As far as actual checks I dont believe so. You are enabling it regardless so it takes that as you have done the checks to make sure everything is correct. On mode does not send messages back and forth like LACP and PAGP. 

If you have mismatched configurations and you bundle the ports you could see some weird forwarding issues and possible spanning tree loop.

Hope that helps

-David

[Joseph W. Doherty]

My understanding is the same as @David Ruess .

[paul driver]

Hello

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@PrimeYeti wrote:

Does an EtherChannel perform checks on a new interface or an opposite switch even if it is enabled manually? E.g. not using LACP or PaGP. I would assume that it does but I couldn't find a clear answer.

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My understanding -Yes it does.
The port channel does not need to be enabled with any link aggregation control for the switch to perform inconsistency detection, meaning the PC can be a static/unconditional PC.
As long as a switchport is assigned to the etherchannel it should check if there is any inconsistency from its far end switch connection by the sending and receiving of the bpdu from its single stp instance.

if the far end switch connection is not replicated into a PC then depending on which far end switchport is designated to reply to a received bpdu the etherchannel switch should detect an inconsistency for which I understand this inconsistency check is actually enabled by default.

[PrimeYeti]

Got a couple of different answers here

Now thinking about it, I guess it wouldn’t perform the checks dynamically since, as David mentioned, the switches will only do checks between each other over PAGP and LACP. 

Still unsure though, are the checks performed on new interfaces being added to an EtherChannel on a single switch with PAGP or LACP disabled? Since it would just be the one switch. Would it add it and disable the interface or just not add it at all if it fails the checks?

[David Ruess]

The "checks" you are referring to is the LACP and PAGP messages sent back and forth with port information. If they dont match then they dont get bundled. For "on" mode you just need the local ports to have the same config and they will bundle regardless. Here is a visual from a lab I have. The other side is not configured at all and the ports still get bundled.

-David

[Joseph W. Doherty · Accepted solution]

Ah, sorry, slightly misread your OP.

You've asked about "checks", in manual mode, both same side and regarding far side, correct?

For far side, again I believe @David Ruess and I think alike, i.e. Etherchannel doesn't do any kind of "checks", regarding the Etherchannel itself, with the far side.  I.e. it doesn't "care" how the far side is configured, again for just Etherchannel.

Near/same side, as you add links, Etherchannel may "check" speed and duplex matches on interfaces.  I recall, on the 6500, by default, Etherchannel would also (same side) determine if different line cards had similar queuing architectures (although also recall, this could be overridden).

As others have already noted, in manual mode, since the two sides don't share Etherchannel information, you can muck up your network if done incorrectly.

As an aside, I'll also mention, over the years, Etherchannel configs sometimes took several config tries to get the Etherchannel to come up correctly.  Often on the larger chassis switches, you had to configure some options on the physical interfaces, some on the logical port-channel interface, some same options on both.

Over the years, Etherchannels accepted more and more configuration statements on just the port-channel interface.

Once you got Etherchannel to come on-line, it working correctly didn't seem to be impacted by the difficulty getting it on-line in the first place.

[David Ruess]

@PrimeYeti As @Joseph W. Doherty mentioned and I left out in my response the same side will be checked so you won’t be able to add mismatched parameters to the link. (You can add it but it will put the port in suspended mode until it’s corrected). This was also tested in my lab. As Joseph also mentioned it is much easier to get port channel working these days with their expanded support/config.

-David

[MHM Cisco World]

this is the LAG flow. 

LACP automatically configures an administrative key value equal to the channel group identification number on each port configured to use LACP. The administrative key defines the ability of a port to aggregate with other ports. A port's ability to aggregate with other ports is determined by these factors:

A-Port physical characteristics, such as data rate, duplex capability, and point-to-point or shared medium

B-Configuration restrictions that you establish

i.e. the LAG check Key-ID which is auto config and contain the physical Characteristics of Ports member. if the Key is same then the port is add to LAG if not it will not add.
https://www.cisco.com/c/en/us/td/docs/ios/12_2sb/feature/guide/gigeth.html

MHM

[Gopinath_Pigili]

Yes it does.

PAgP and LACP; both are negotiation protocols that dynamically configure an Etherchannel. PAgP is a Cisco proprietary protocol so you can only use it between Cisco devices. LACP is an IEEE standard which many vendors support.

If you are going to create an EtherChannel you need to make sure that all interfaces have the same configuration:

Duplex.
Speed.
Native and allowed VLANs.
Switchport mode (access or trunk).

PAgP and LACP will check if the configuration of the interfaces that you use is the same.

It’s also possible to configure a static EtherChannel without these protocols doing the negotiation of the link for you.

Best regards
******* If This Helps, Please Rate *******

[paul driver]

Hello

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@PrimeYeti wrote:
Now thinking about it, I guess it wouldn’t perform the checks dynamically since, as David mentioned, the switches will only do checks between each other over PAGP and LACP.
Still unsure though, are the checks performed on new interfaces being added to an EtherChannel on a single switch with PAGP or LACP disabled?

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If you are on about actual configuration checks then without lacp/pagp misconfiguration isn’t checked-However as i have already mentioned etherchannel inconsistencies are detected on aggregation links by default without any link aggregation control feature.
Review this:

[PrimeYeti]

Hi Paul,

Seen, thank you for providing this. I hadn't even thought about EtherChannel inconsistency at this point.

Kind regards,

Connor

[Joseph W. Doherty]

Great reference, however don't overlook that reference only mentions support on some Catalyst switches and appears dependent on STP which might be disabled or not supported (for the latter, like on routers and perhaps on L3 switches where port-channel interface is L3).