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QOS on the 100 series unmanaged switches

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May 22, 2012 7:57 AM
May 22nd, 2012

Hi jeremy

Thought I would answer the question here regarding QOS features within the 100 series switch family.

Lets see what I can give hopefully it's not too confusing. I grabbed the following from a internal discussion on QOS on 100 series switches.

Hey,  we are talking about unmanaged switches, so QOS has to be hardwired with hopefully optimum with  pre-defined settings.

Because these are Cisco Small Business switches, you will see references below to LVS phone RTP.  As an example, the  silicon within the switches will honor  and queue according to  priority settings on our SPA phones.

QoS Support with the 100 series unmanaged switches - some rules.


Unmanaged switches supports egress queuing based on  DSCP and 802.1p , such that,

  • for up to 8 port switches,  upon receiving Tagged frame with 802.1p bits and a DSCP, the switch will choose use the 802.1p value to assign the packet  to one of two egress queues..
  • for 16 oports and above, upon receiving a tagged frame with 802.1p bits and DSCP  the switch will use DSCP to assign to one of four egress queue.  This is the reverse behaviour when compared to 8 port switches.
  • For untagged packets, any  DSCP value is used to  to assign the packet to a egress queue.
  • The switches within this family also incorporate Priority queue with weighted-round-robin (WRR).

So in summary;

Up to and including eight port switches

Umanaged up to 8 ports,  will look at 802.1p bits and ignore  DSCP values if both are present.  With untagged packets, the switch prioritizes based on DSCP values.

Sixteen port switcehs and above

On sixteen port and above  switches,  will look at DSCP values and ignore  802.1p CoS  values, if both are present.. With untagged packets, the switch prioritizes based on DSCP values.



Following chart shows DSCP to egress queue mapping  for switches with,  or more than 16 ports..


pve.jpg


Now lets look at the smaller unmanaged switches.

SF100D-08 supports in silicon,  7 predefined DSCP values set to High priority queue:  DSCP values 46 (EF), 10 (AF11), 18 (AF21), 26 (AF31), 34 (AF41), 48 (CS6), and 56 (CS7).

Ok what does this mean, these 8-port  unmanaged switches recognizes only 7 predefined DSCP values and  802.1p,  but has only two egress queues per port. If it sees these matching  DSCP value and no 802.1p bits,  it will queue these packets into a higher priority queue, else all traffic will use a lower priority egress queue.  If it sees a tagged packet with  these matching DSCP values but also a 802.1p but, it will ignore the DSCP values.

Which every way you look  at it,   there is a way to differentialte and actually utilize some QOS or COS  in a unmanged switch. cool stuff



pve1.jpg



pve2.jpg

Note:

  • As said further up in this posting, SF100D-08/P supports 2 egress queues whose WRR weight assignment 1:16 (6%-94%), as seen from the normal or High settings in the table above.
  • Queues shown as 1,2,3,4 in the table above,   reference the default WRR behaviour for  sixteen port switches and above.

IPv6 support

This switch is a transparent bridge to IPv4 and IPv6 packets.

The  switches support DSCP values from IPv6 packets, except SF100D-08/P

(SF100D-08/P switches ignore the value of the DSCP on the IPv6 packets and take untagged-IPv6 as default priority traffic).

Pretty darn good for a unmanaged product.

regards Dave

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Posted May 22, 2012 at 7:57 AM
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