Issues in Etherchannel

[Mohamed Kabeer S]

Hi all,

I know the concept of etherchannel But I don't know how it works and I want to know how many maximun links are used to bind as single logical links and also how many minimum links are used to bind as single logical link.

Thanks and Regards,

Kabeer

[Peter Paluch]

Hello Kabeer,

EtherChannel works by sending frames over multiple links. When a frame is to be sent through an EtherChannel bundle, a hashing function is performed over the frame's address fields and the result of this hashing points to exactly one of the links in the EtherChannel bundle through which the frame will be subsequently sent. This way, multiple frames with different values in their address fields are distributed among various links in the EtherChannel. Depending on the switch, the hashing may take various addressing fields into account - source, destination or both, MAC, IP, even L4 and port and combinations thereof. Two switches interconnected via an EtherChannel can perform different hashing.

In any case, a single flow of frames (flow being a sequence of frames from a particular source to a particular destination) will end up being sent through a single link in the EtherChannel because the addressing fields of all frames in this flow will carry the same source and destination, and the hashing function will always yield the same result. This is done to prevent frame reordering - a flow of frames carried over a single link will not be subject to possible frame reordering. Thus, a single flow will not experience any improvement in the transmission speed, as it is limited to a single link only. However, because multiple flows will be distributed among different links in the EtherChannel, the aggregate bandwidth of this connection is higher.

Read more about EtherChannels here:

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

EtherChannel supports up to 8 active physical ports in a single bundle, and the minimum number of physical ports to make an EtherChannel work is 1 although having just a single link in an EtherChannel - apart from situations where all other links have failed for some reason - does not make much sense.

Feel welcome to ask further!

Best regards,

Peter

[Joseph W. Doherty]

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Posting

In the "real-world", I just wanted to emphasis, the hashing algorithm being used vs. the traffic crossing the Etherchannel, determines how well your multiple flows "randomly" use your multiple links.  It's not common today, as newer platforms default hashing usually uses multiple frame or packet attributes, but a poor choice can result in most or even all traffic using just one link.

Etherchannel does not react to individual link loading, i.e. one link might be saturated with multiple flows, while another is being unused.

Etherchannel generally hashes better across power of two links, i.e. 2, 4 or 8.  It will distribute traffic across other link numbers, but not proportionally.  (NB: a feature of the rather new Sup2T improves on this.)