QoS (Quality of Service) on Cisco Catalyst switches is not as easy as configuring it on routers. The big difference is that on routers QoS runs in software while on our switches it’s done in hardware. Since switching is done in hardware (ASICs) we also have to do our congestion management (queuing) in hardware. The downside of this is that QoS is in effect immediately. In this article I will give you an overview of all the different commands and an explanation of how QoS works. If you are totally new to LAN QoS do yourself a favor and watch this video:
The first 54 minutes are about classification, marking and policing so if you only care about congestion management and queuing you can skip the first part. Having said that let’s walk through the different commands.
Priority Queue
If your switch supports ingress queuing then on most switches (Cisco Catalyst 3560 and 3750) queue 2 will be the priority queue by default. Keep in mind that there are only 2 ingress queues. If we want we can make queue 1 the priority queue and we can also change the bandwidth. Here’s how to do it:
Switch(config)#mls qos srr-queue input priority-queue 1 bandwidth 20
The command makes queue 1 the priority queue and limits it to 20% of the bandwidth of the total internal ring bandwidth.
For our egress queuing we have to enable the priority queue ourselves! It’s not enabled by default. Here’s how you can do it:
Switch(config)#interface fa0/1 Switch(config-if)#priority-queue out
The command above will enable the outbound priority queue for interface fa0/1. By default queue 1 is the priority queue!
Queue-set
The queue-set is like a template for QoS configurations on our switches. There are 2 queue-sets that we can use and by default all interfaces are assigned to queue-set 1. If you plan to make changes to buffers etc. it’s better to use queue-set 2 for this. If you change queue-set 1 you will apply you new changes to all interfaces.
This is how you can assign an interface to a different queue-set:
Switch(config)#interface fa0/2 Switch(config-if)#queue-set 2
Above we put interface fa0/2 in queue-set 2. Keep in mind that we only have queue-sets for egress queuing, not for ingress.
Buffer Allocation
For each queue we need to configure the assigned buffers. The buffer is like the ‘storage’ space for the interface and we have to divide it among the different queues. This is how to do it:
mls qos queue-set outputbuffers Q1 Q2 Q3 Q4
Above you see the mls qos command. First we select the queue-set and then we can divide the buffers between queue 1,2,3 and 4. For queue 1,3 and 4 you can select a value between 0 and 99. If you type 0 you will disable the queue. You can’t do this for queue 2 because it is used for the CPU buffer. Let’s take a look at an actual example:
Switch(config)#mls qos queue-set output 2 buffers 33 17 25 25
This will divide the buffer space like this:
- 33% for queue 1.
- 17% for queue 2.
- 25% for queue 3.
- 25% for queue 4.
Besides dividing the bufferspace between the queues we also have to configure the following values per queue:
- Threshold 1 value
- Threshold 2 value
- Reserved value
- Maximum value
The command to configure these values looks like this:
mls qos queue-set outputthreshold T1 T2 RESERVED MAXIMUM
First you need to select a queue-set, select the queue number and finally configure a threshold 1 and 2 value, reserved value and the maximum value.
Here’s an example:
Switch(config)#mls qos queue-set output 2 threshold 3 33 66 100 300
In the example above we configure queue-set 2. We select queue 3 and set the following values:
- Threshold 1 = 33%
- Threshold 2 = 66%
- Reserved = 100%
- Maximum = 300%
This means that threshold 1 can go up to 33% of the queue. Threshold 2 can go up to 66% of the queue. We reserve 100% buffer space for this queue and in case the queue is full we can borrow more buffer space from the common pool. 300% means we can get twice our queue size from the common pool.
Assign marked packets/frames to correct queue
You now know how to configure the buffers and thresholds but we still have to tell the switch which CoS and DSCP values have to go to which queue. Here’s the command for it:
mls qos srr-queue
This is what it means:
- Direction: input or output.
- Marking: CoS or DSCP.
- Queue: The queue number.
- Threshold: this can be threshold 1,2 or 3.
- Values: The CoS or DSCP values you want to put here.
Let’s take a look at an actual example:
Switch(config)#mls qos srr-queue output cos-map queue 1 threshold 1 0 1
The command assigns CoS values 0 and 1 to queue 1 up to threshold 1.
Switch(config)#mls qos srr-queue output cos-map queue 1 threshold 2 2 3
This example assigns CoS values 2 and 3 to queue 1 up to threshold 2.
Switch(config)#mls qos srr-queue output cos-map queue 4 threshold 2 6 7
And this one assigns CoS values 6 and 7 to queue 4 up to threshold 2.
Bandwidth Allocation
The buffers determine how large the queues are. In other words how ‘big is our storage’. The bandwidth is basically how often we visit our queues. We can change the bandwidth allocation for each interface. Here’s what it looks like for our igress queuing:
mls qos srr-queue input bandwidth Q1 Q2
Igress queuing only has two queues. We can divide a weight between the two queues. Here’s an example:
Switch(config)#mls qos srr-queue input bandwidth 30 70
With the command above queue 1 will receive 30% of the bandwidth and queue 2 will receive 70%. These two values are “weighted” and don’t have to add up to 100%. If I would have typed somethine like “70 60″ then queue 1 would receive 60/130 = about 46% of the bandwidth and queue 2 would receive 70/130 = about 53%. Of course it’s easier to calculate if you make these values add up to 100.
For our egress queues we have to do the same thing but it will be on interface level. We can also choose between shaping or sharing. Sharing means the queues will divide the available bandwidth between each other. Shaping means you set a fixed limit, it’s like policing. Here’s an example:
Switch(config)#interface fa0/1 Switch(config-if)#srr-queue bandwidth share 30 20 25 25
This will divide the bandwidth as following:
- Queue 1: 30%
- Queue 2: 20%
- Queue 3: 25%
- Queue 4: 25%
In this case we have a 100Mbit interface which means queue 1 will receive 30Mbit, queue 2 20Mbit, queue 3 25Mbit and queue 4 25Mbit. If there is no congestion than our queues can go above their bandwidth limit. This is why it’s called “sharing”.
If I want I can enable shaping for 1 or more queues. This is how you do it:
Switch(config)#interface fa0/1 Switch(config-if)#srr-queue bandwidth shape 20 0 0 0
This value is a weighted value. The other queues are not shaped because there’s a 0. When you configure shaping for a queue it will be removed from the sharing mechanism. So how much bandwidth does queue 1 really get? We can calculate it like this:
- 1/20 = 0.05 x 100Mbit = 5Mbit.
So traffic in queue 1 will be shaped to 5Mbit. Since queue 1 is now removed from the sharing mechanism…how much bandwidth will queue 2,3 and 4 get?
Let’s take a look again at the sharing configuration that I just showed you:
Switch(config)#interface fa0/1 Switch(config-if)#srr-queue bandwidth share 30 20 25 25
I just explained you that queue 1 would receive 30Mbit, queue 2 20Mbit, queue 3 25Mbit and queue 4 also 25Mbit. Since I enabled shaping for queue 1 it doesn’t join the sharing mechanism anymore. This means there is more bandwidth for queue 2,3 and 4. Here’s what the calculation looks like now:
- Interface fa0/1 is 100Mbit.
- We configured shaping to 5Mbit for queue 1 so there is 95Mbit left.
- We configured a weighted of value 20,25 and 25 for queue 2,3 and 4.
- 20 + 25 + 25 = 70 total.
- Queue 2 will receive 20/70 = 0.28 * 100 Mbit – 5 Mbit = 27.1 Mbit.
- Queue 3 will receive 25/70 = 0.35 * 100 Mbit – 5 Mbit = 33.9 Mbit.
- Queue 4 will receive 25/70 = 0.35 * 100 Mbit – 5 Mbit = 33.9 Mbit.
If we add all these values together:
- Queue 1 is shaped to 5 Mbit.
- Queue 2 is shared to 27.1 Mbit.
- Queue 3 is shared to 33.9 Mbit.
- Queue 4 is shared to 33.9 Mbit.
- 5 + 27.1 + 33.9 + 33.9 = Total bandwidth of 100 Mbit.
That’s how you do it!
It’s also possible to rate-limit the entire interface for egress traffic if you want to save the hassle of configuring shaping. This is how you do it:
Switch(config)#interface fa0/1 Switch(config-if)#srr-queue bandwidth limit 85
This will limit our 100 Mbit interface to 85% so you’ll end up with 85 Mbit.
Verification and troubleshooting
Now you know how to configure everything. Let’s take a look at the different commands you can use to verify everything!
First you should check the capabilities of a switch. You can do this on the interface level as following:
Switch#show interfaces fa0/23 capabilities FastEthernet0/23 Model: WS-C3560-24PS Type: 10/100BaseTX Speed: 10,100,auto Duplex: half,full,auto Trunk encap. type: 802.1Q,ISL Trunk mode: on,off,desirable,nonegotiate Channel: yes Broadcast suppression: percentage(0-100) Flowcontrol: rx-(off,on,desired),tx-(none) Fast Start: yes QoS scheduling: rx-(not configurable on per port basis), tx-(4q3t) (3t: Two configurable values and one fixed.) CoS rewrite: yes ToS rewrite: yes UDLD: yes Inline power: yes SPAN: source/destination PortSecure: yes Dot1x: yes
Above you can see that this Cisco Catalyst 3560 switch has 4 queues with 3 threshold levels.
If you are configuring QoS you need to make sure you enabled it globally first with the “mls qos” command. You can verify if QoS is active or not with the following command:
Switch#show mls qos
QoS is enabled
QoS ip packet dscp rewrite is enabled
It tells us that QoS is enabled globally. We can also check the QoS parameters for each interface as following:
Switch#show mls qos interface fa0/1 FastEthernet0/1 trust state: trust cos trust mode: trust cos trust enabled flag: ena COS override: dis default COS: 1 DSCP Mutation Map: Default DSCP Mutation Map Trust device: none qos mode: port-based
Above you can see the trust state for this interface. We can also verify the queue-sets for this switch. If you didn’t configure them you will find some default values:
Switch#show mls qos queue-set Queueset: 1 Queue : 1 2 3 4 ---------------------------------------------- buffers : 25 25 25 25 threshold1: 100 200 100 100 threshold2: 100 200 100 100 reserved : 50 50 50 50 maximum : 400 400 400 400 Queueset: 2 Queue : 1 2 3 4 ---------------------------------------------- buffers : 33 17 25 25 threshold1: 100 33 33 100 threshold2: 100 66 66 100 reserved : 50 100 100 50 maximum : 400 200 200 400
Above you will find queue-set 1 and 2. You can see how the buffers are divided per queue and the values for our thresholds, reserved and maximum values.
We can check how queuing is configured per interface. This is how you do it:
Switch#show mls qos interface fastEthernet 0/24 queuing FastEthernet0/24 Egress Priority Queue : disabled Shaped queue weights (absolute) : 25 0 0 0 Shared queue weights : 25 25 25 25 The port bandwidth limit : 100 (Operational Bandwidth:100.0) The port is mapped to qset : 1
Above you see that the priority queue is disabled. Also you see the shaped and shared values and that this interface belongs to queue-set 1.
If you are troubleshooting you should check if you see any drops within the queues. You can do it like this:
Switch#show platform port-asic stats drop FastEthernet 0/1 Interface Fa0/1 TxQueue Drop Statistics Queue 0 Weight 0 Frames 0 Weight 1 Frames 0 Weight 2 Frames 0 Queue 1 Weight 0 Frames 0 Weight 1 Frames 0 Weight 2 Frames 0 Queue 2 Weight 0 Frames 0 Weight 1 Frames 0 Weight 2 Frames 0 Queue 3 Weight 0 Frames 0 Weight 1 Frames 0 Weight 2 Frames 0
Here you can see the drops for each queue. We can also verify if we are receiving traffic that is marked:
Switch#show mls qos interface fastEthernet 0/1 statistics FastEthernet0/1 (All statistics are in packets) dscp: incoming ------------------------------- 0 - 4 : 0 0 0 0 0 5 - 9 : 0 0 0 0 0 10 - 14 : 0 0 0 0 0 15 - 19 : 0 0 0 0 0 20 - 24 : 0 0 0 0 0 25 - 29 : 0 0 0 0 0 30 - 34 : 0 0 0 0 0 35 - 39 : 0 0 0 0 0 40 - 44 : 0 0 0 0 0 45 - 49 : 0 0 0 0 0 50 - 54 : 0 0 0 0 0 55 - 59 : 0 0 0 0 0 60 - 64 : 0 0 0 0 dscp: outgoing ------------------------------- 0 - 4 : 0 0 0 0 0 5 - 9 : 0 0 0 0 0 10 - 14 : 0 0 0 0 0 15 - 19 : 0 0 0 0 0 20 - 24 : 0 0 0 0 0 25 - 29 : 0 0 0 0 0 30 - 34 : 0 0 0 0 0 35 - 39 : 0 0 0 0 0 40 - 44 : 0 0 0 0 0 45 - 49 : 0 0 0 0 0 50 - 54 : 0 0 0 0 0 55 - 59 : 0 0 0 0 0 60 - 64 : 0 0 0 0 cos: incoming ------------------------------- 0 - 4 : 2 0 0 0 0 5 - 7 : 0 0 0 cos: outgoing ------------------------------- 0 - 4 : 0 0 0 0 0 5 - 7 : 0 0 0 Policer: Inprofile: 0 OutofProfile: 0
That’s all I have for you for now! I suggest you to check out these commands on your own switches to become familiar with them.
