Questions on Switch Port Aggregation with EtherChannel

Answered Question
Jul 19th, 2010

Hi guys,


I was reading through the topic on Ether-Channels, and I'm quite confused, and need your guys guidance..


taken from a book im reading,



For example, an EtherChannel consisting of two links bundled together  requires a 1-bit
index. If the index is 0, link 0 is selected; if the  index is 1, link 1 is used. Either the lowestorder address bit or the  XOR of the last bit of the addresses in the frame is used as the index. A  four-link bundle uses a hash of the last 2 bits. Likewise, an  eight-link bundle uses
a hash of the last 3 bits. The hashing  operation’s outcome selects the EtherChannel’s outbound link

As  an example, consider a packet being sent from IP address 192.168.1.1 to  172.31.67.46.
Because EtherChannels can be built from two to eight  individual links, only the rightmost
(least-significant) 3 bits are  needed as a link index. From the source and destination
addresses,  these bits are 001 (1) and 110 (6), respectively. For a two-link  EtherChannel,
a 1-bit XOR is performed on the rightmost address bit:  1 XOR 0 = 1, causing Link 1 in the
bundle to be used. A four-link  EtherChannel produces a 2-bit XOR: 01 XOR 10 = 11,
causing Link 3 in  the bundle to be used. Finally, an eight-link EtherChannel requires a
3-bit  XOR: 001 XOR 110 = 111, where Link 7 in the bundle is selected.


From what I have understand... it is possible for us to build and configure EtherChannels from 2 to 8 individual links.


My question is..


1) So it is possible to build Etherchannels with 3, 5, 6 or 7 individual links?


2) if so,  a two-link etherchannel , they uses a 1-bit index... and only the rightmost  least significant bits are needed as link index.


- for a four-link etherchannel, they uses a 2-bit, and for an eight-link bundle uses 3bits as the index...


so in that case, how many bits do etherchannel uses for a 3-link, 5-link, 6 and 7-link bundles?



please kindly direct me in the right direction ... thanks...


Regards,

Jason

Correct Answer by Reza Sharifi about 6 years 7 months ago

Hi Jason,


Yes, you can have 3, 5 or 7 links in an Etherchannel, but the distribution is not going to get done evenly.  When building Etherchannels, It is recommend to configure them with the power-of-2.  So 2, 4 or 8.  Etherchannel has 8 "buckets" to distribute among member ports.

So a 2 port bundle have 2 4 port buckets and a 4 port bundle has 4 2 port buckets and 8 port bundle has 8 1 port bucket, but for example an uneven bundle of 3 would have 2 3 port buckets and 1 2 port buckets.


HTH

Reza

Correct Answer by Nagaraja Thanthry about 6 years 7 months ago

Hello,


For combinations about 2 links i.e. 3 through 8, the switch uses 3 bits. The

switch will allocate the states such that all hashes have a corresponding

link identified. This is similar to a situation where you have an 8 port

etherchannel and one of the ports went down. Please check the following

document:


http://cisco.biz/en/US/tech/tk389/tk213/technologies_tech_note09186a0080...

14.shtml


Hope this helps.


Regards,


NT

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Correct Answer
Nagaraja Thanthry Mon, 07/19/2010 - 06:53

Hello,


For combinations about 2 links i.e. 3 through 8, the switch uses 3 bits. The

switch will allocate the states such that all hashes have a corresponding

link identified. This is similar to a situation where you have an 8 port

etherchannel and one of the ports went down. Please check the following

document:


http://cisco.biz/en/US/tech/tk389/tk213/technologies_tech_note09186a0080...

14.shtml


Hope this helps.


Regards,


NT

Correct Answer
Reza Sharifi Mon, 07/19/2010 - 07:27

Hi Jason,


Yes, you can have 3, 5 or 7 links in an Etherchannel, but the distribution is not going to get done evenly.  When building Etherchannels, It is recommend to configure them with the power-of-2.  So 2, 4 or 8.  Etherchannel has 8 "buckets" to distribute among member ports.

So a 2 port bundle have 2 4 port buckets and a 4 port bundle has 4 2 port buckets and 8 port bundle has 8 1 port bucket, but for example an uneven bundle of 3 would have 2 3 port buckets and 1 2 port buckets.


HTH

Reza

naturezlove Mon, 07/19/2010 - 21:50

hey NT and Rezal,


Thanks for the explanation and link.. makes clearer sense to me now .. cheers! ^^



Regards,

Jason

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