Nagendra Kumar ... Mon, 12/28/2009 - 23:11

Hi,

Backplane on a switch is the fabric bandwidth between ports. This allows traffic within the device from one port to other. For example, if you have 5 ports on a switch and if traffic needs to flow from port 1 to port 5, it will be moved through backplane from port1 to port5.

32/256/720 specifies the available bandwidth.

On a 6500 switch, based on Supervisor engine and Line Card, your backplane bandwidth will be 32 or 256 or 720 Gbps.

For example, If you have Sup720, possible backplane speed is 720 Gbps (Not for each Line Card) and shared 32 Gbps (for legacy line cards). When CEF720 line card is inserted on a slot, it will be connected to the backplane @ 20 Gbps unidirectional or 40 Gbps bidirectional.

So on a 6509 switch with one SUP720 (no redundant), you are left with 8 slots for line card. Assuming all line cards are CEF720, you will have 40 Gbps per slot which is 8*40 Gbps = 320Gbps.

If you have legacy line cards like 6148, it will be connected to the shared 32 Gbps and not with Supervisor fabric 720 Gbps backplane.

If my memory serves right, stacking on 3750 series switch will provide you 32 Gbps.

HTH,

Nagendra

Giuseppe Larosa Tue, 12/29/2009 - 01:18

Hello Alsayed,

as explained by Nagendra a switching fabric allows for transferring of frames between different port pairs at the exact same time.

When this is possible in any condition the switching fabric is defined not blocking.

In a shared bus architecture that we can find for example on an old C7500 the single frame is moved from source port to destination port using a shared resource.

So a backplane is very important for a LAN switch to provide very high performances, also for example regarding latency and jitter that are less dependent from overall system load then in a shared bus architecture.

About C750 stacks:  the stackwise cables implements a ring topology with speed 32 Gbps for standard 3750 and speed 64 Gbps for 3750E. So it is important to note that without using the stackwise cables and using standard GE ports to connect them you have a so called daisy chain with much less performance.

Hope to help

Giuseppe

Peter Paluch Tue, 12/29/2009 - 04:20

Hello,

In addition to Nagendra's and Giuseppe's wonderful answers, the stacking on 3750 and 3750E also makes the several physical switches to appear as a single logical switch with a larger number of ports. Thanks to the stacking, all the physical switches are configured from a single place (the so-called stack master) and to the outside world, they appear as a single switch. The StackWise technology is therefore not just a high speed data interconnect, but rather it also provides functions to cluster the switch stack into a single logical entity. That would not be possible with a simple GigE interconnection between two switches (without using additional protocols as in VSS).

Best regards,

Peter

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