Etherchannel PaGP VS LACP

[jk865]

Hi 

I was wondering if somebody would be kind enough to clarify my understanding on the concept of Ether channel and let me know if I'm missing anything important. 

• Ether channel combines multiple physical links into one logical link (port channel) to circumnavigate STP , Ether channel can be used to increase bandwidth and provide redundancy without the need for additional equpment 
• it supports up to 8 connection 
• Configuration must be the same on both sides 
• PAgP is ciscos version LAGP is the IEEE standard version 
• Port modes must be either on, on , desirable ,desirable, auto, desirable for the connection to establish. 
• LACP on, on , active, active or passive active. 
• The modes don't make any difference as long as they match? 

Thanks 

[MHM Cisco World]

• Ether channel combines multiple physical links into one logical link (port channel) to circumnavigate STP , Ether channel can be used to increase bandwidth and provide redundancy without the need for additional equpment <<-correct
• it supports up to 8 connection <<- depend on platform 
• Configuration must be the same on both sides <<- there are some config not must to be same like SW priority 
• PAgP is ciscos version LAGP is the IEEE standard version  <<- Correct 
• Port modes must be either on, on , desirable ,desirable, auto, desirable for the connection to establish. <<- see below note
• LACP on, on , active, active or passive active. <<- see below note 
• The modes don't make any difference as long as they match? <<- see below note 
  
  
  

[balaji.bandi]

Both work as expected, PAGP cisco prop means you need to have both the side Cisco device, and the LACP open standard can be used from vendor to vendor.

small compare from my notes :

More information can be found here :

https://www.learncisco.net/courses/icnd-2/etherchannel-and-l3-redundancy/understanding-etherchannel.html

[David Ruess]

Most of that is pretty accurate.

As far as the 8 connections those would be active but I think you can have up to16 in the bundle, so if a link fails it would just be replaced by one of the standby links in the group.

Etherchannel does not increase BW. Bundling 2 - 1 gigabit links together does not give you 2 gigabit speed. You still have 2 1 gigabit speed links that share a path.

Etherchannel increases the lanes on the highway but does not increase the speed of the cars.

Depending on your level of study or wanting to understand you can look into the load balancing mechanism used (you would determine that on a case by case basis for you environment). You could also play with the LACP priority for the primary/secondary switch election - this determines what ports get bundled if there are more than the allowed 8, and which one is next if a link fails.

-David

[Joseph W. Doherty]

In addition to the great information provided by the other posters . . .

"to circumnavigate STP"

Yes and no.  Between links in the bundle, yes, between the rest of the topology, no, as STP often runs across Etherchannel too.

"Ether channel can be used to increase bandwidth"

Another yes and no.  (David's example of adding highway lanes - great!)

With Etherchannel, any one flow is still limited by physical individual link bandwidth, as a single flow always only uses just one link.

However, other flows can used other links, concurrently.  So you do have more aggregate bandwidth, just as a highway with more lanes can move more traffic.

Although David mentions load balancing as a level of study, to actually effectively use Etherchannel, you need to understand load balancing options, and your traffic.  If not, you might have all your traffic continue to use only (just) one link (although you will always have redundancy [if there's more than one workable link in the bundle]).

Also understand, although there's more aggregate bandwidth, Etherchannel, even with best LB choice, is "dumb".  Two flows can be directed to the same link, while another link is totally unused.  Again, even with best LB, for a dual Etherchannel link, you might likely only average about a 50% boost, not 100%.

"without the need for additional equpment"

Correct, if both sides' equipment supports your bundling technology, and you have the "spare" ports.

"PAgP is ciscos version LAGP is the IEEE standard version"

Actually, it's the converse.  Cisco's had the technology first, but considered it proprietary (so it required Cisco hardware).  So, industry/IEEE seeing a "good thing" created their own "standard" variant.  (Cisco has a lot of "first" proprietary technologies that a later industry/standard variant was defined to provide something similar.)

"The modes don't make any difference as long as they match?"

I believe it depends on the actual modes, what combinations work together (or not - which can be intended).

BTW, although Etherchannel is still great for redundancy, from a strictly bandwidth perspective, sometimes the next hardware bandwidth "step up" is a better choice.  Using the highway analogy, the "speed limit" is increased.  (Also, in some situations, like working with fiber and optical transceivers, a single faster "faster" link can be less expensive than multiple Etherchannel links that don't even match the "faster" link's bandwidth.)