Implementing QoS on 3750

[Atif Masood]

I have to implement QoS on 3750 switch.

I have two types of traffic received on that switch.

I have two incoming links both of 10Mb. The outgoing link is 10Mb as well. I have high priority and low priority traffic coming to switch that passes through the outgoing10 Mb link until it’s not congested but if I receive more traffic then it prioritises high priority traffic over the lower priority traffic and drop (or queues) the lower priority traffic.

For Example,

Case 1: If Router at Link A sends 8Mb high priority traffic and Router at Link B sends 6Mb low priority traffic, then Switch forwards 8Mb traffic to Outgoing Link as its High Priority and send low priority 2Mb traffic from Link B and drop (or queues) the remaining Link B low priority traffic.

Case 2: Similarly If Router-Link A sends 10Mb hjigh priority and Router-Link B sends low priority 6Mb traffic, SW should forward only Link A 10Mb traffic and drops (or queues) Link B traffic as outgoing link is only 10Mb.

Case 3: If Router-Link A sends 8Mb High Priority and 1 Mb Low Priority traffic and Router-Link B sends 1 Mb High Priority and 5 Mb low priority traffic SW should forward 9Mb High Priority and 1 Mb Low Priority (Random) and Queue remaining low priority (or drops it). 

Can anyone help me how should I implement this by explaining configurations for 3750?

Note:

I marked high priority traffic with ip precedence 5 at routers, configured mls qos trust ip precedence and priority-queue out on 3750 switch. This solution didn't worked for me.

P.S: I have attached Scenario Diagram as attachment.

[Richard Bradfield]

Hi Mat,

The problem with the 3750 is that you can only add policy-maps to the input of an Interface see link below.

http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/91862-cat3750-qos-config.html

so from Router A and B you can classify the traffic as required on the input interfaces of the switch. But you can't put a policy map on the output of the Interface that goes to Router C.

so then you need to put the policy map on the outside interface of Router C, you can do the input to router C but you can really only police the input, not so much control as doing it on the output interface.

HTHG

Richard

[Joseph W. Doherty]

"This solution didn't worked for me."

And you know this how?  (Reason I ask, from what you've described, unless incorrectly configured, it should work.)

[Atif Masood]

On router A:

policy-map test

class class-test

set ip precedence 5

int gi0/0

service-policy input test

SW:

mls qos

int Gi1/0/1

mls qos trust ip-precedence

priority-queue out

Router C: Configs for monitoring

class-map match-all PREC5

match ip precedence 5

!

policy-map test

class PREC5

!

int Gi0/1

service-policy input test

I used CMA3000 to generate 10Mb traffic from both routers Link A and B. 

At the other router the results I am getting are 5.8 Mb from Router 1 (High Priority) and 4.2 Mb from Router(Low Priority) where actually I should get 10 Mb from Router 1 only.

I am not sure what's wrong? Is there some buffers to play some role in it as well? or may b I am genarating constant traffic CMA3000 Traffic generator?

[Joseph W. Doherty]

Could you post another diagram, clarifying the test topology and what interfaces are connected to what?  Also clarify what your class maps are matching.

[Atif Masood]

I have attached the complete Scenario Diagram.

At Router A, I have created a Service Policy (using Policy maps and class maps) to set the traffic an IP Precedence 5.

I generated 10 Mb traffic from Router A and 10 Mb traffic from Router B which is low priority traffic and no marking is done.

At Switch 3750 I have used the following commands:

mls qos trust ip precedence

priority-queue out

I believe that by issuing this command It should trust IP Precedence 5 traffic and put it in queue 1 (PQ) and forward this traffic and since at the receiving end it just have a 10 Mb Profile which will allow this traffic and remaining low priority traffic will be dropped.

I have created class maps at receiving router C just to match IP Prec 5 traffic so I can identify how much traffic I received and after doing this I found out I am receiving 5.8 Mb IP Prec 5 traffic while 4.2Mb traffic from Router B where I was expecting I should be getting 10Mb IP Prec 5 traffic and no traffic from Router B.

[Joseph W. Doherty]

You've enabled QoS, globally, on the 3750?

You're using QoS defaults (especially mappings)?

You've configured all ports with trust IP and PQ out?

The 3750 egress port (that connected to rightmost L2 switch) is physically running at 10 Mbps?

Why does your sketch show packet drops at (rightmost) L2 switch?

[Atif Masood]

Yes QoS is enabled globally on 3750 and all ports are configured to trust IP Prec and PQ.

No Speed is set manually on 3750 Egress port and its a gig port.

All the packet drops are done at Layer 2 Switch (That Layer 2 Switch is actually Arista Switch which has different cards installed on it including video cards ( for HD, SD and ASI) and data cards (on which profiles of different BW can be set) so Its set to 10Mb and all packet drops done there. 

At the Receiving Router I can see both traffics (IP Prec 5 and RTR B). 

[Joseph W. Doherty]

"No Speed is set manually on 3750 Egress port and its a gig port."

Set to what speed?

[Atif Masood]

I believe its running on full 1G however I can confirm that when I will be in lab tomorrow.

[Joseph W. Doherty]

If it is, try running it at 10.

[Atif Masood]

It is already set to 10 Mb.

I also removed all the Switches inbetween and now setup is simply

Router A ---- 3750---- Router B

But still same results.

[Joseph W. Doherty]

If top right link was running at 10 Mbps, why does your scenario.jpg show drops on top right switch's ingress interface?

If all the L2 switches have been removed, where do you see drops now?
 

[Atif Masood]

It was initially set to Auto Auto which is setting it to 100Mb so all the drops were at L2 SW.

I removed all the Layer 2 inbetween switches and set interface speed to 10 Mb between Router B and 3750 and drops at 3750 egress interface but still PQ didn't work I am getting the same results.