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How is rate-limit implemented on EXOS?

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TitleHow is rate-limit implemented on EXOS?
Objective
How is rate-limit implemented on EXOS platforms?
Environment
  • EXOS all
  • Summit X440, X460, X480, X650, X670
  • Black Diamond 8900-MSM128, 8900-Series I/O modules
  • Black Diamond X8 
Procedure
To process one packet, 64000 tokens are required.
Examples:
Rate-limit value has been configured of 262144 pkts/sec.


Description: Description: Description: Description: cid:image002.png@01CF4982.6F3DF020
  
Hence in one second, 16777216000/64000 -> 262144 packets will be processed and the remaining packets will be dropped.
 
Test Result:

Port      Link        Rx Pkt      Rx Byte       Rx Pkt       Rx Pkt   Flood Rate
          State        Count        Count        Bcast        Mcast     Exceeded
================================================================================
5         A             1000         2080            0            0          819
5         A              100         6400            0            0           79
 
Configured rate-limit value = 262144
 
Example 1:

 
5         A             1000         2080            0            0          819
 
The above packets are sent at the rate of 1,488,095 pkts/sec. (Line rate 1GE)
The time to transmit one packet at this rate is 1/1,488,095
secs  -> 0.672 microseconds. Scaling the rate to 1000 packets gives, 1000/672  pkts/microseconds.
Number of meter refreshes took place at this time interval
-> 672/15.625 -> 43.008 -> 44 times      
Number of tokens generated via 43 meter refreshes
-> 44 * 262144 -> 11534336 tokens.       
So
number of packets that can be processed in 672 microseconds -> 11534336/ 64000 -> 180 packets  
Additional 1 packets are processed in all scenarios, because of buffer.
Totally 181 packets are processed and 819 packets are dropped.

 
Example 2:
 

5         A              100         6400            0            0           79

Scaling the rate 1/0.672 pkt/microseconds to 100 packets, it is 100/67 pkts/microseconds.
Number of meter refreshes took place at this time interval 
-> 67/15.625 -> 4.288 -> 5 times
Number of tokens generated via 5 meter refreshes 
-> 5 * 262144 -> 1310720 tokens.
So
number of packets that can be processed in 67 microseconds -> 1310720/64000 -> 20 packets.
Totally 21 packets are processed and 79 packets are dropped.
 
However when the configured rate-limit value is very less (lets say 1, 2), two packets are allowed to get processed.
Additional notes

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