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why only eight ports can be aggregated to form ether channel

binojbaby1984
Level 1
Level 1

Is there any particular reason to limit the maximum number of ports to 8  in an ether channel .  Could any one can explain the reason for that

3 Replies 3

Peter Paluch
Cisco Employee
Cisco Employee

Hi,

My personal take on this is that it does not really make much sense to allow for more than 8 ports in a single EtherChannel, as the speed of individual Ethernet variants goes in multiples of 10. If you need to bundle more than 8 ports running on, say, 1 Gbps, then you're certainly better off using a 10 Gbps interface.

It also seems to be related to Cisco's original implementation of the hashing function that performs the actual per-frame link selection in an EtherChannel bundle. Certainly, it does not make sense to have more links than the number of values this hashing function can produce, as some links would never be used in this case. Because hashing functions generally express their output using a fixed number of bits, you can have a hashing function produce 1-bit result (0 or 1 = 2 values), 2-bit result (0 to 3 = 4 values), 3-bit result (0 to 7 = 8 values), and so on. Clearly, when you think of this, having a hashing function provide only 2 bits of output would allow you to have only 4 links at maximum in an EtherChannel, which is rather limiting - and in addition, the distribution of the traffic would be rather crude because each value of this hashing function represents 1/4 of the total volume, that is, 25%. If you had only 3 links in an EtherChannel, one of them would need to handle two values of the hashing function, and so the load distribution would be 50% : 25% : 25%, clearly a very skewed distribution. From this viewpoint, the choice of a 3-bit hashing function, and allowing at most 8 links at an EtherChannel, seems like a reasonable compromise: Allowing more links does not really make sense as in such case, it is better to look for a faster Ethernet variant, and the traffic is distributed across links in amounts of 1/8 of the total traffic, or 12.5%, which is kind of acceptable, even if not that fine-grained. Hence the 8 as the maximum number of links

Just for your information, some new Catalyst and Nexus platforms are told to use an 8-bit result hash value, generating 256 different values. This is not to allow for 256 ports in an EtherChannel - it would make no sense considering the impractical nature of allocating more than 8 ports for a single interconnection and the fact that using a faster Ethernet version would in general produce better results. However, using 256 different output values allows for much fine-grained traffic distribution among the links: Each value represents 1/256, or 0.390625% of total traffic. So if you had 3 links, each of the links would handle 256 DIV 3 = 85 values of the hashing function, and 256 MOD 3 = 1 link would handle an additional hashing value (quick check: 85 x 3 + 1 = 255 + 1 = 256). In this case, the load distribution would be 86:85:85, or roughly 33.6% : 33.2% : 33.2%. Compare this to a 3:3:2 distribution with a 3-bit hashing function producing 8 results, or roughly 37.5% : 37.5% : 25% - clearly a much better improvement in smoothing out the traffic.

You're welcome to ask further!

Best regards,
Peter

I don't know if there is a "technical" reason or why you would want to do that, it's just the standard.

Joseph W. Doherty
Hall of Fame
Hall of Fame

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It likely has something to do with the fact that you can have 8 values in 3 bits, and someone, at some point thought 2 bits didn't offer enough, and 4 bits seemed excessive.

I know it can be hard to understand now a days, but once upon a time, "bit" space was expensive, so deciding on how many bits to represent a value was often a difficult decision.

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