Etherchannel loadsharing

[udo.konstantin]

Hi all,

Is it possible to have also an unequal number of ports in an etherchannel?

For example I have 7 ports on an channel...what happens about loadsharing ?

I heard about loadsharing is only possible with even port numbers (2,4,6,8)?!

Thanks

Udo

Sent from Cisco Technical Support iPad App

[catapia84 · Accepted solution]

Hi

An equal balance can be obtained only at 2,4, and 8

With other combination you can get :

Number of Ports in the EtherChannel  ---   Load Balancing

8 ---    1:1:1:1:1:1:1:1

7 ---    2:1:1:1:1:1:1

6 ---    2:2:1:1:1:1

5 ---    2:2:2:1:1

4 ---    2:2:2:2

3 ---    3:3:2

2 ---    4:4

If you have 7 ports, one of them will be around 90% of load,  the rest will be around at 50%

Hope this will be useful for you, and hope you mark as correct answered

Regards!

[Joseph W. Doherty]

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Posting

Carlos Tapia Sanchez wrote:
Number of Ports in the EtherChannel  ---   Load Balancing7 ---    2:1:1:1:1:1:1If you have 7 ports, one of them will be around 90% of load,  the rest will be around at 50%

For 7 wouldn't it be 2/8 for one link, i.e. 25%, and 1/8, i.e. 12.5%, for each of the other links?

[catapia84]

Hi Joseph

It is at the rate of 2 for 1 , while you have an amount of traffic on the interface, you must have "the double" in the other one.

For example in 6 :

6 ---    2:2:1:1:1:1

Your Interfaces, must act like this :

30%   30%  15% 15% 15% 15%

or

70% 70%  35% 35% 35% 35%

all depends the amount of total traffic.

Let me know your comments!

Regards!

[Joseph W. Doherty]

Disclaimer

The     Author of this posting offers the information contained within this     posting without consideration and with the reader's understanding  that    there's no implied or expressed suitability or fitness for any   purpose.   Information provided is for informational purposes only and   should not   be construed as rendering professional advice of any kind.   Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In     no event shall Author be liable for any damages whatsoever   (including,   without limitation, damages for loss of use, data or   profit) arising  out  of the use or inability to use the posting's   information even if  Author  has been advised of the possibility of  such  damage.

Posting

Yes agree the ratio is 2:1 between some ports, but the total load sharing, I believe, you need to account for all the other ports.

In your new example for 6 ports, your percentages exceed 100%.

For 6 ports, a "1" port should carry 1/(2+2+1+1+1+1) of total capacity, or 1/8 or 12.5%.

A "2" port (for 6) should carry 2x that or 25% of total capacity.

For 2 ports, your 4:4 = 4/(4+4) or 1/2 or 50%.  (NB: 4:4 is also 2:2 or 1:1)

[catapia84]

Hi Joseph

Sorry if I´m not clear in my last comment.

When I say :  30%   30%  15% 15% 15% 15%

I´m talking about each Interface capacity.

This matches with your comment :

-----------------------------------------------------------------------------------------------------------------

For 6 ports, a "1" port should carry 1/(2+2+1+1+1+1) of total capacity, or 1/8 or 12.5%.

A "2" port (for 6) should carry 2x that or 25% of total capacity.

-----------------------------------------------------------------------------------------------------------------

If you have "X" amount of traffic on each of four Interfaces corresponding to rate [1], in my example : 15%

You always must have "2X" in the other two Interfaces that correspond to [2] , in my example the resulting is 30%

I focus in the amount of traffic of each Interfaces, because for example if you have an Etherchannel of 6 Gig Interfaces,     at the moment when you have 4 Gigs of traffic, two of the Interfaces will be at 100% an the rest 4 will be at 50% of usage., and this may cause some packet drops.

Talking about 4 Gigas is the 100% of traffic,  you say that the "2" Interfaces must have 25% ,  and the "1" Interfaces must have 12.5%

This results on 25%  of 4G  = 1 Giga        represents  100% of the Interface

                       12.5%  og 4G= 500Mb      it represents 50% of the Interface.

You are not even in 5 Gigas of total traffic, when you start to have problems, due to the load of the [2] Interfaces.

I hope I was clearer this occasion.

Regards!

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

I'm sorry Carlos, but I'm still a bit baffled by your percentage usages and have a different understanding of actual link loadings.

On the latter, let's take the simple example of dual channel Etherchannel.  It doubles our capacity, as each link carries half the load, right?  Nope, because individual flows are not split across links.  Whether two links or eight links, the fastest transfer rate of a single flow will be the capacity across a single link.

Instead of one flow we use two, also across two links, now we send twice as much, right?  Nope, as each flow is assigned to a link statistically "randomly" (and proportionally for the non-power of twos links).  The 2nd flow might be assigned to the the link already being used or it might be assigned to the free link.  A 3rd flow might also be assigned to the same link as the 1st two flows, or it might be assigned to the free link.  Statistically, as you increase the number of flows you'll likely distribute them across you're multiple links.

Also Etherchannel doesn't account for actual link loading, so if you have two busy flows and one light flow, the two busy flows could compete for the bandwidth of one link while the other's is underutilized.

More links just allow "random" distribution across additional paths.  The non-power of two Etherchannels combinations just mean that some links should be assigned, over time, more flows.

In your example of 4 Gig across 6 gig links, statiscally we should see links 1 and 2, over time, carring more flows which often will also imply higher traffic too.  However, at any specific point in time, 4 gig of aggregate traffic might transit the 6 links without any issues while at another time 2 gig of aggregate traffic might have congestion issues.  Whether 4 gig or 2 gig aggregate I can only tell you statically what your link loadings might be, cannot tell you what they will be.

[Peter Paluch]

Hi Udo,

In addition to a nice answer from Carlos, this is the URL of the document where this and other facts about EtherChannel load sharing mechanisms are nicely described, including specialties of different platforms:

http://www.cisco.com/en/US/tech/tk389/tk213/technologies_tech_note09186a0080094714.shtml

Best regards,

Peter