Loading Balancing on Cisco IOS

[network_geek]

Hi!

Can anyone explain to me how do cisco devices router or switched do load balancing of bundled interfaces (bunde-ether,port-channel)?

[Georg Pauwen]

Hello,

on port channels, you have the options below for load balancing. Source or destination MAC address, or both, source or destination IP address, or both, source or destination port, or both:

port-channel load-balance { src-mac | dst-mac | src-dst-mac | src-ip | dst-ip | src-dst-ip | src-port | dst-port | src-dst-port }

[network_geek]

Hi Greg! Thank you for your reply. I appreciate that. However, my question is how does LACP or PAGP does load balancing by default? What is the algorithm it uses?

[Joseph W. Doherty]

Defaults, I recall, vary per actual device (series). (Also not all switch series support the same hashing options.) I believe src-mac was a common default, but again, I recall it varies.

[network_geek]

Thank you, Joseph. Do you have any document that might give me further insight into this?

[Joseph W. Doherty]

I have a very hazy recollection of once seeing some Cisco document that may have listed, at the time, what Etherchannel hash options different switches supported. Don't recall if it listed defaults.

You should be able to determine the default, and hash options, by looking at the IOS (command usage) documentation for each device in question. (Yea, not convenient if there's no summary document.)

[network_geek]

Hi Joseph!
Thank you for the document. I am looking for something that might help me understand the algorithm of the hashing.

[Joseph W. Doherty]

If you're looking for exactly how Cisco hashes, i.e. the actual hashing algorithm, I believe it's considered proprietary information.

[network_geek]

I understand. You mean to say there is no way we can know how data destined for a destination is load-balanced across an aggregated interface?

[Joseph W. Doherty · Accepted solution]

For yet as of unseen traffic, correct (at least such is my understanding). For traffic that's already once been subjected to the hashing algorithm, "like" (i.e. with same hashed attribute contents) traffic, it will always take the same link (for that algorithm, and that number of links).

BTW, for CEF load-balancing across multiple links, I believe there's a command you can use to feed it the attributes of the packet and it will inform you what llnk will be used. Again, don't recall there being a L2 Etherchannel similar command.

If you're wondering why this is proprietary, well a good hashing algorithm isn't always trivial to define, and considering it's likely "baked" into a L2 switch's hardware, it has value to Cisco to keep it to themselves. (I.e. Cisco may do the hash distribution "better" than Brand X.)