Guaranteed bandwidth

Unanswered Question
Jun 28th, 2007


I have a number of switches in my network, and a number of VLANs running across each. I want to guarantee 1 particular VLAN is given priority when traffic is heavy, to ensure quality of the connection.

The switches are 3560s.

How do I go about this?

I am thinking of setting:

mls qos cos 4

on all my important VLAN ports,

mls qos cos 2

on the rest; except trunk, which I set

mls qos trust cos

Is this correct?

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 4 (1 ratings)
Pavel Bykov Thu, 06/28/2007 - 06:28

Recommended QoS practice is to deploy it end to end. Your proposed setting will only affect output queues of the interface on this one particular switch. In any case, you should at least define which COS belongs to what queue using

mls qos srr-queue output cos-map queue X threshold X COS-NUMBER

cisco-pix Thu, 06/28/2007 - 07:42

The line you suggested adding: is that a global config command?

I assume the same line needs set on each switch?

Amit Singh Thu, 06/28/2007 - 08:31

Along with setting the COS on the switch ports, you have to assin the bandwidth to the internal switchport queues for handling the traffic with the priority. By default if you have the COS value set as the priority 5, the last port queue will become the strict priority queue and will get the guranted bandwidth of 40 % on the ports. You have to allocate the bandwidth on the switchports as well using the command " wrr-queue bandwidth ".

You have to map the proper COS value to the respective queue so that it gets the guaranted bandwitdh using the command "wrr-queue map cos " command. You can also map the COS to DSCP vlaues and then trust the dscp vlaue on the trunk ports if you want the end-end QOS to be extended over the WAN.

Please see the link below for QOS config on 3560 switches:


-amit singh

cisco-pix Thu, 06/28/2007 - 09:58

Checking the default values, I see on Ingress traffic in Queue 1 gets 90% buffer, 2 gets 10%. 2 also gets higher priority.

This suits my needs - I have a connection which will have little traffic but needs priority.

Would it therefore be suitable to leave qos as default, and set my priority ports to cos value = 5 so it goes into queue 2, and all other ports cos=4?

I do not want to overcomplicate it. All I have is 4 switches which are trunked together using fiber: some ports on 2 of the switches will need that priority I talk of above.

Is this right?


johnnylingo Tue, 07/24/2007 - 16:16

I don't see any problem with that. On the 3560, Ingress Queues go 90/10, and Egress goes 25/25/25/25. Traffic that is CoS 5 goes in to Q2 of the Ingress Queue and Q1 of Egress. You might want to use "priority-queue out" to make sure Q1 on the Egress side gets prioritized.


This Discussion