Etherchannel

[MK31]

If a frame travels on this EtherChannel link, will it be transmitted over both Ethernet cables?

[Richard Burts · Accepted solution]

No a frame would not be transmitted over both cables. It would be transmitted on only one cable. The EtherChannel has logic to select which cable to use for a particular frame (applies to both layer 2 EtherChannel and layer 3 EtherChannel). There are several parameters that can be considered in making that choice and there are some configuration options available to alter which parameters are used in the decision.

[balaji.bandi · Accepted solution]

As per cisco :

EtherChannel reduces part of the binary pattern that the addresses in the frame form to a numerical value that selects one of the links in the channel in order to distribute frames across the links in a channel. EtherChannel Frames are forwarded over an EtherChannel link that is based on the results of a hashing algorithm. The algorithm is deterministic; if you use the same addresses and session information, you always hash to the same port in the channel. This method prevents out-of-order packet delivery.

Good reference document : ( i always use)

https://www.cisco.com/c/en/us/support/docs/lan-switching/etherchannel/12023-4.html

[Joseph W. Doherty]

BTW although @balaji.bandi provides an excellent reference, be aware it's not current.

For example, doing a quick scan of the document, don't believe I noticed it mentioning that some (later) platforms have increased the size of the internal hash value to better distribute traffic across multiple links.

Or, I don't believe it addresses what happens if Etherchannel links are added or removed while traffic is using it.  (A frame will not be split across links, ever, but, I believe, a flow's packets might start using a different link.  The latter, if it happens, might result [briefly] in some out-of-order frame/packet delivery.)