• 40G TapAgg BiDi Design

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Arista QSFP BiDi optics allow both transmit and receive signals, to travel on the same fiber slice. The QSFP BiDi design provides 40Gbps (20Gbps egress & 20Gbps ingress on each fiber slice) bandwidth using the duplex MMF (OM3, & OM4) cable. This way one can leverage the existing cabling infrastructure to upgrade BW from 10Gbps to 40Gbps ( see diagram 1).

The BiDi optics provides a number of benefits and some are listed below:

   Fiber replacement not required when moving from 10Gbps to 40Gbps

   Lower CAPEX as 75% less fiber comparing parallel cable (40G SR4)

   Interoperable with Industry standard 40G QSFP+ BiDi

   All Arista QSFP+ ports supported

   SRBD optics support distance 100m (OM3) and 150m (OM4)


10Gto40G  Diagram 1: 10G to 40G upgrade using existing fiber  

 Platform compatibility (TapAgg)

 Platforms support TapAgg functionality.






Software Support

The support for command “unidirectional receive-only” is available in EOS4.20 release. This command is required on TapAgg interface (BiDi Optic) to disable transmit signal on fibers.

Other software features are not in the scope of this post.

40G Tap-Agg Design Consideration

40Gbps TapAgg infrastructure can be built by either parallel (SR4) or Duplex (BiDi) fiber plant. The BiDi (Duplex) provides benefits over SR4 (Parallel) as described in earlier section so BiDi is preferred.

While using BiDi optical transceiver for Tap ports, it is important to have a good understanding of fiber signal budget, and distance for both production and TapAgg devices. The passive taps are available in different split ratios: 50/50 , 60/40, 70/30, 80/20. These ratios represent signal distribution between the tap and switch. The tap split ratio should be carefully selected keeping in view the signal strength and distance of the end devices connected to the fiber. Any carelessness in selecting tap ratio may result in communication loss.

It is important to know that for a BiDi optic design, only two ports on TapAgg device are required for full data feed reception. Arista support interoperability with industry standard 40G BiDi taps.


Diagram 2: 40G BiDi Transceiver Tap & TapAgg Connectivity

Above diagram shows standard tap connectivity and traffic flow directions.


As it is visible from the diagram 2 that TapAgg ports will receive data on both transmit and receive fiber. It is required that “unidirectional receive-only” feature should be enabled on all TapAgg tap ports so that BiDi transceiver do not generate transmit signal. 

interface e1
           unidirectional receive-only

 The sample configuration is shown above.

For more information please visit the Arista documentation at – “https://www.arista.com/en/support/product-documentation” 


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