9 Replies Latest reply: Sep 7, 2010 4:57 AM by Scott Morris - CCDE/4xCCIE/2xJNCIE RSS

    bandwidth of switch port

    A.N

      HI

       

       

                                in switch  when we use command " sh interface <interface name> " . we see that Bandwidth is 100 Mbs.  means  every port can transfer data at 100 Mbps . suppose 24 port are in switch  in all port host are connected.Can Every host  send data 100 Mbs at a time??? . can switch handel this.

       

       

       

       

       

       

      regards

       

      Ankush

        • 1. Re: bandwidth of switch port
          Erick

          You bet it can.  I recommend you do some research on ASICs.

           

          Good luck.

           

          Erick

          • 2. Re: bandwidth of switch port
            Paul Stewart  -  CCIE Security

            In most cases all ports cannot transmit at 100Mbp/s at the same time for two reasons.  First, is that most switches have an oversubscribed backplane.  So once the aggregate throughput reaches the speed of the backplane, that is the maximum that can be transmitted per second.  The second reason is that load distribution is not usually so evenly distributed that the more important ports (server, uplinks, etc) are not oversubscribed before other ports are.

            • 3. Re: bandwidth of switch port
              A.N

              hi

               

               

                                      Thanks for reply.. one more thing  if i change bandwidth of port  using  bandwidth command.   it really effect the traffic. suppose if i do

              1 kbps . is it really host can not send that data above 1 kbps .

               

               

               

               

               

              regards

               

              ankush

              • 4. Re: bandwidth of switch port
                Paul Stewart  -  CCIE Security

                Changing the bandwidth is only changes the reference point for stuff like qos.  Changing the "speed" actually forces the port to operate at a given speed (i.e. 1000, 100, or 10).

                • 5. Re: bandwidth of switch port
                  Scott Morris - CCDE/4xCCIE/2xJNCIE

                  PacketU wrote:

                   

                  In most cases all ports cannot transmit at 100Mbp/s at the same time for two reasons.  First, is that most switches have an oversubscribed backplane.  So once the aggregate throughput reaches the speed of the backplane, that is the maximum that can be transmitted per second.  The second reason is that load distribution is not usually so evenly distributed that the more important ports (server, uplinks, etc) are not oversubscribed before other ports are.

                   

                  A very good point, and important thing to pay attention to.  But not always true.

                   

                  Some line cards/switches ARE capable of every port working at wirespeed simultaneously without killing anything.  You need to look at the port:asic ratio.  If an ASIC is controlling 6 ports, that means that combined, those 6 physical ports can send at 100M (or 1000M depending on switch port speed), and it really doesn't matter which port does which, but all at the same time won't happen.

                   

                  Cisco has switches/linecards that work at 8:1, 6:1, 4:1, 2:1 and 1:1 (there used to be some 12:1, but I think they're all gone now).  Just be aware of what specific device you are putting into your network design and what your specific intention (use) needs to be for them!

                   

                  Remember, there's the whole idea of statistical multiplexing (odds of all devices transmitting at full speed all the time at the same time) that often "hides" this disparity.  And MOST people really don't need to worry about it in their networks because of those statistics.  But IF you really need all ports at full speed (givin' her all she's got, Captain), then be prepared to pay for it!

                   

                  As Paul states though, plan your bottlenecks appropriately!  100 workstations with 1gig ports trying to go out a 10M internet connection sucks if anyone tries to go full speed. 

                   

                  Scott

                  • 6. Re: bandwidth of switch port
                    Brian

                    Yes, most new Cisco switches (2950, 2960, 3550, 3560, 3570) have a 32Gbps switching backplane, and the 24-port 10/100/1000 Mbps versions of these popular switches can handle this load with no problem.  The 48-port versions will not (48Gbps all ports vs 32Gbps backplane).  The Cisco 6500 series can have switching backplanes as high as 720 Gbps.  However, as both Scott and Paul alude to, the chances of all ports operating at full wirespeed at the same time at any given time, is highly unlikely.  When planning your switching design, pay attention to what Scott was saying about bottlenecks.  In regards to planning any network design, remember the 3-Ps, Proper, Planning Prevents, Pi$$, Poor, Performance.

                     

                    HTH

                     

                    • 7. Re: bandwidth of switch port
                      Scott Morris - CCDE/4xCCIE/2xJNCIE

                      With the 6500, you even have to look at linecard specifics...

                       

                      the 6148, for exam is NOT  a line-rate card.  The 6748, however, is.

                       

                      Scott

                      • 8. Re: bandwidth of switch port
                        Paul Stewart  -  CCIE Security

                        Also, with the 3750 and the 3750-X, my understanding is every frame hits the stack ring.  So it is the stack that is limited to 32Gbps or 64Gbps respectively.

                        • 9. Re: bandwidth of switch port
                          Scott Morris - CCDE/4xCCIE/2xJNCIE

                          The stack ring is merely an extension of the backplane, so that would stand to reason.  I'm not entirely sure that EVERY frame has to traverse it though if locally terminated, although I could be wrong on that.   I've never really sat to think about it since the 32G (64G if you are doing marketing math) is significantly faster than any etherchannel we could create there anyway, so I never really cared. 

                           

                          Scott