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Hashing algorithm for link aggregation groups (LAGs) in an EX8200 Virtual Chassis / XRE200 environment



Article ID: KB22976 KB Last Updated: 13 Feb 2012Version: 1.0
KB22943 explains the hashing algorithm used for determining the outgoing member link in a link aggregation group (LAG) for EX Series Switch platforms. This KB explains a difference in EX8200 Virtual Chassis / XRE200 platforms, known as 'local link bias'.
Virtual Chassis platforms consist of more than one physical entity (chassis) and link aggregation groups (LAGs) connecting to these platforms typically terminate links in different chassis for physical redundancy. However, as switching platforms grow, crossing internal links add latency and increases the utilization of these links. The generic EX Series Switches LAG hashing algorithm does not take into account the location of the member links in the LAG and determines the outgoing member based purely on the information in the frame/packet header. Applying the same algorithm to the EX8200 Virtual Chassis/XRE200 platform would result in unnecessary latency and possible congestion of internal virtual chassis links.

EX8200 Virtual Chassis / XRE200 platforms introduce a new feature as part of the LAG hashing algorithm. This feature is known as 'local link bias'. In summary, the algorithm only uses LAG member links connected to the same chassis as the incoming interface of the frame/packet. This is best explained with a diagram:

  • In the example above, EX4500-0 is connected to the EX8200VC on ports xe-4/0/0 and xe-20/0/0 (ae0); and EX4500-1 is connected to the VC on ports xe-7/0/0 and xe-23/0/0 (ae1).
  • The linecards in the EX8200VC are connected using four links: xe-2/0/0, xe-2/0/1, xe-4/0/4 and xe-4/0/5.
  • Traffic destined from EX4500-0 to EX4500-1 ingressing the EX8200VC will always use xe-7/0/0 as the outgoing interface and never use xe-23/0/0, as this would require the traffic to cross from EX8216-0 to EX8216-1 using one of the four links between the linecards.
  • Generic LAG hashing algorithm detailed in KB22943 applies for multiple local links on the same chassis or multiple remote links.
  • The local link bias feature will soon be available in EX4200/EX4500 as a user configurable knob. The feature is in implementation stage at the time of this writing.

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