link aggregation

[Brahim dias]

1. What is "mode on"? What are its advantages and disadvantages?

2. If there are 2 physical links, each running at 1 GB, will they be seen as 2 GB on the logical level when configured as a port channel?

3. Why is it not recommended to configure interfaces that are not a power of 2 in Link Aggregation?

4o mini

[marce1000]

      >What is "mode on"? What are its advantages and disadvantages?
      "mode on" means always do link aggregation on the configured interfaces without negotiation regardless
      of whether the remote partner wants it or not : disadvantages are , 1) Not negotiating lacp parameters 
     2) Unwanted induced traffic at the partner side (e.g.)

      >If there are 2 physical links, each running at 1 GB, will they be seen as 2 GB on the logical level when configured as a port channel?
      The term logical level does not apply , one could say that there could be an overall throughput of 2 GB

     >Why is it not recommended to configure interfaces that are not a power of 2 in Link Aggregation?
       Because of hashing algorithms that work better when the number of links involved is a power of 2

 M.

[Joseph W. Doherty]

@marce1000 didn't see your reply until after I posted mine.

"Unwanted induced traffic at the partner side"

Indeed!  Which can have some very nasty effects, like L2 loops.

"The term logical level does not apply , one could say that there could be an overall throughput of 2 GB"

I recall (?) a port-channel show interface showing the aggregate bandwidth of its active ports and for something like OSPF's auto costing, unless bandwidth or OSPF cost were manually configured, OSPF interface cost would change as physical port-channel links went up/down.

Assuming my recollection is correct, or even if not, I would think one could say there is a logical aggregate bandwidth.  Otherwise, a port-channel load can exceed 100% load unless the aggregate value is used.

What you won't see is any individual flow exceeding one member link's bandwidth.

[Joseph W. Doherty]

#1 "mode on" activates Etherchannel, on that device, without any agreement with the other side.  I recall, way, way back it was initially the only option and/or the only option on some devices even when protocols like PAgP or LACP became common.

Advantages and disadvantages very similar to other technologies that are just either on or off without any dynamic setup or monitoring.  For example, hard coded speed and/or duplex vs. auto negotiation for copper Ethernet ports or static routing vs. dynamic routing.

#2 Usually yes.

#3 Usually because simple binary hashing was used and port usage ratios, for non binary multiples were not very equal.  Later, on at least one platform (6509?) Cisco much improved the distribution ratios on non binary multiples.

[Joseph W. Doherty]

BTW, for # mod 8 link ratios:

Number of Ports in the EtherChannel Load Balancing

8 1:1:1:1:1:1:1:1

7 2:1:1:1:1:1:1

6 2:2:1:1:1:1

5 2:2:2:1:1

4 2:2:2:2

3 3:3:2

2 4:4

However for comparison, # mod 256 (for just 2 to 4 links):

4 64:64:64:64

3 86:85:85 <<== much better!

2 128:128

[M02@rt37]

Hello @Brahim dias 

"Mode On" in link aggregation is a static method where multiple physical links are manually combined into a single logical link without using dynamic protocols like LACP or PAgP. Its main advantage is simplicity and immediate availability, as it does not rely on negotiation processes. However, it lacks dynamic management and error detection, meaning it cannot automatically adjust to link failures or recover from them, which can lead to potential network issues. When two 1 Gbps links are aggregated in this manner, they can be seen as a single 2 Gbps logical link, but the bandwidth utilization depends on the hashing algorithm used for traffic distribution. It is generally not recommended to use a number of interfaces that are not a power of 2 in link aggregation because it can lead to uneven load balancing and suboptimal use of available bandwidth.

[MHM Cisco World]

1. What is "mode on"? What are its advantages and disadvantages? Never use mode ON in PO between two SW, mode ON use only between SW and device not support lacp 

2. If there are 2 physical links, each running at 1 GB, will they be seen as 2 GB on the logical level when configured as a port channel? Yes correct 

3. Why is it not recommended to configure interfaces that are not a power of 2 in Link Aggregation? you misunderstand this, the cisco recommend use number of port member in PO as number power of 2 to make load balance more accurate.
   i.e. you can use 2 ports in PO or use 4 in PO or use 8 in PO 
   this recommend not mandatory