Etherchannel: Load Balancing

Etherchannel increases the available bandwidth by carrying multiple frames over multiple links.  A single frame is always transmitted across a single link in an Etherchannel bundle when traffic enters a switch a hashing function is performed on the address fields in the frame producing a number that identifies which link the frame will transmit over.

The sequence of frames having the same value in the address fields fed into the hashing function is called a conversation or a flow.  The hashing function is deterministic, meaning that all frames in a flow will produce the same hash value, therefore forwarding of the frames in a flow will all traverse a single link and will not be per packet forwarded across multiple links.

This allows for multiple flows to traverse multiple links increasing higher aggregate throughput.  This also prevents frames from being received out of order.

You want to ensure there is a mix of frames sent over a port channel, usually access ports are sending frames from hosts who are transmitting only to their default gateway, and vice versa on return traffic, gateway sourced traffic is being sent back to the unique hosts.  To mix this up you could balance based on source MAC address for outgoing traffic, and then balance using destination MAC on the return traffic, so there are unique flows generated for the hash value and its not all the same going over a single link.

The maximum number of member links in a bundle is 8, the hashing function produces a 3 bit result in the range of 0-7 whose values are assigned to individual member links.   If there are fewer physical links some of the links will be assigned multiple values from this range and ultimately those links will carry more traffic than others as a result.

Traffic will only be distributed evenly across multiple links in a bundle when the number of links equals 8, 4, or 2.



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