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Nexus Fabric Modules

Nexus Fabric Modules

Posted on Jan 23, 2020 (0)

Nexus Fabric Modules

Fabric Modules are like backplane of nexus Switches, when all the Line cards, Supervisor are inserted to nexus Chassis, then all line cards and supervisor modules are connected to each other via fabric Module. Traffic between Ingress and egress line card must pass through Fabric modules.

Fabric Module are like Meeting point of all line cards and supervisor modules.

Nexus 7000 Supported fabric Modules are given below:

Below is the detailed description of Nexus 7700 supported fabric Module and its BW capacities.

Below is the detailed connection details of  Nexus 7000 fabric Module. Where each module is connected to each fabric module via two traces and each supervisor is connected to fabric module via one trace.


Nexus 7000 Fabric Module System Calculation

Three stage for Fabric Modules Nexus 7000 is given below:

Here the traffic enter at (stage-1) then go to fabric module (stage -2) and the go to egress module (stage -3)

Now we can also see the Fabric model internal connection on nexus 7004 Chassis.  In fabric ASIC is present in Supervisor engine and each line cards are connected to each supervisor with 2 Fabric channel of 55 GB each.

VOQ: 

Virtual output queuing (VOQ) is a virtual queuing technique in which instead of single queue for all traffic, separate queues are maintained for each possible output location. It also solves a common problem known as head of line blocking.

In VOQ, at input port it's physical buffer maintains a separate virtual queue for each output or egress port. Therefore if any congestion are there on egress port packet will reside at virtual queue of each egress port . Other packets in the same physical buffer destined to different (non-congested) output ports are in separate virtual queues and can therefore still be processed.

Problem: HOL (Head of Line Blocking): Suppose there is a road, which connect to a stadium and a mall, and cars are going to both location, Please refer the below picture for more understanding, according the below picture

Grey cars are going towards stadium and Red cars are going towards mall, but due to single lane, Red card has to wait for its front cars to clear the lane then only they can reach to mall.

Now the solution for this issue is to create the two separate QUEUES, for Grey cars and Red Cars. But we  still have a problem , that what if the traffic towards stadium is huge  which caused the stadium full , then in that case , all Grey cars which on still on its lanes have to moved back .

Another solution is to introduce the Arbiter , so any  cars ( Gery or Red ) wants to go to Stadium or mall , has to take permission to Arbiter , if the Arbiter find that space are there to accommodate the traffic , it will allow to enter the cars ( traffic ) in to that queue else it ask  for wait .

Egress Buffers:

Egress Buffer are those virtual buffers which are 4 in priority (0-3) per Port Group in Line cards. If line cards is of 24 ports and it supports the Grouping of 4 ports then there will be 8 groups of even and odd numbers of ports and each group will have 4 queue with priority (0 to 3). Please refer the below picture for more clarity.

VOQ Operation:

Following are the steps used by VOQ for proper working of its operation.

Step 1: In the First step, each module inform the supervisor about the egress buffer available to them for each port group as well as they will share their own egress buffer information to each ingress module

Step 2: When a frame is to be switched between modules, let say from module 1 port 1/1 to module 2/3 port then the packet has to cross the fabric module (backplane). In this case ingress module queue the frame to VOQ of module 1 and request permission to transmit the frame over crossbar fabric to supervisor.

Step 3: if the egress buffer is available for port e2/3 then supervisor will grant permission to ingress module and deduct one credit from available egress buffer. Now frame is then transmitted from ingress module to egress module (port 2/3 ) via  cross bar fabric.

When the egress module  forwards the frame to physical port then it notifies to supervisor that egress buffer for that port is available .

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