Controller Based WLANs

Why the traffic is not evenly distributed across the port channel members?
Problem:

1. We have configured a port channel on the Aruba controller. However we see that all the member links are not utilized equally. What could be the reason behind this?

 



Diagnostics:

An Aruba controller allows 8 port channels with maximum of 9 members in a single port channel. Aruba controller calculates hash of a flow and uses that hash to load balance the traffic the across the member ports. It calculated the hash based on the following:

 

1. For an IP traffic (ether type is 0x0800), it uses source and destination IP addresses. 

2. For non-IP traffic, it uses source and destination mac addresses. 

 

We can see the port utilization statistics using the command below:

 

#show port stats

Port Statistics
---------------
Port      PacketsIn  PacketsOut  BytesIn   BytesOut  InputErrorBytes  OutputErrorBytes  CRCErrors
----      ---------  ----------  -------   --------  ---------------  ----------------  ---------
GE 0/0/0  42855      17035       10275992  3906829   0                0                 0
GE 0/0/1  15907      16022       2727570   3175533   0                0                 0
GE 0/0/2  0          0           0         0         0                0                 0
GE 0/0/3  0          0           0         0         0                0                 0
GE 0/0/4  0          0           0         0         0                0                 0
GE 0/0/5  0          0           0         0         0                0                 0
PC 2      58762      33057       13003562  7082362   0                0                 0
 

 #show datapath lag

Datapath Trunk (Port-Channel) Table
-----------------------------------
Port-Channel  Ports   Hash Distribution
------------  ------  -----------------
2             0/0/0   0, 2, 4, 6
              0/0/1   1, 3, 5, 7
 

We see that the port channel 2 above has two members GE 0/0/0, 0/0/1. 



Solution

1. We must make sure that the port channel uses EVEN number of ports rather than odd. For an ODD number of ports, it will not be possible to have an equal hash distribution:

 

 #show datapath lag

Datapath Trunk (Port-Channel) Table
-----------------------------------
Port-Channel              Ports                    Hash Distribution
------------                      ------                       -----------------
1                                 0/0/0                    0, 2, 4, 6
                                   0/0/1                    1, 3, 5, 7

 

Above, we see that the both port GE 0/0/01 and GE 0/0/2 have 4 hashes in the table. For a port channel with 3 member ports, we see:

#show datapath lag

Datapath Trunk (Port-Channel) Table
-----------------------------------
Port-Channel            Ports                  Hash Distribution
------------                     ------                    -----------------
1                               0/0/0                      0, 3, 6
                                 0/0/1                      1, 4, 7
                                 0/0/2                      2, 5

 

We can see above the port 0/0/2 has only two hashes assigned to it thus it will be used less as compared to first two ports which have 3 hashes assigned to them.


2. Port channel load balancing algorithm can only decide the Egress port. Uplink device may send the return traffic back using its own algorithm. Aruba has no control over traffic received. Thus a links may not be equally utilized for incoming traffic. 

3. Aruba will accept the return traffic even if it not received on same link as it was sent. 

4. We also must make sure that the uplink device uses similar load balancing algorithm as Aruba. Please refer to your switch vendor documentation for the same. 

Version history
Revision #:
2 of 2
Last update:
‎02-23-2017 03:48 PM
Updated by:
 
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