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3750 QoS

Posted on 2014-01-12
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Last Modified: 2014-01-29
I have QoS configured on several 3750s. I have Cisco VoIP phones connected to the switches. I also have voice vlan configured on the access switches.
My questions are follow:
- I have the QoS commands below at the global configuration mode. What happens if I don't have those QoS commands? Why do you need cos to dscp mapping?
mls qos map cos-dscp 0 8 16 24 32 46 48 56
mls qos srr-queue output cos-map queue 1 threshold 3  5
mls qos srr-queue output cos-map queue 2 threshold 1  2 4
...
- I have the command below on the interface. Why do I need to mark the packets as the Cisco phone already marked the packets and I use voice vlan?
policy-map lan-in
  class voice
   set dscp ef
  class callsig
   set dscp cs3
...
interface FastEthernet1/0/1
 switchport access vlan 10
 switchport mode access
 switchport voice vlan 20
 service-policy input lan-in
 speed 100
 duplex full
 srr-queue bandwidth share 1 70 25 5
 srr-queue bandwidth shape  30  0  0  0
 priority-queue out

- Do I have to have the two srr-queue bandwith commands below on int fa1/0/1
interface FastEthernet1/0/1
...
 srr-queue bandwidth share 1 70 25 5
 srr-queue bandwidth shape  30  0  0  0
...
0
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Question by:leblanc
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8 Comments
 
LVL 45

Expert Comment

by:Craig Beck
Comment Utility
To put it simply COS to DSCP conversion is important because COS is marking traffic at Layer2, but when a packet traverses a router the packet is rewritten with the DSCP marking.

So,

COS = Layer2
DSCP = Layer3
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Author Comment

by:leblanc
Comment Utility
What if I don't have these commands on the global interface?
mls qos map cos-dscp 0 8 16 24 32 46 48 56
mls qos srr-queue output cos-map queue 1 threshold 3  5
mls qos srr-queue output cos-map queue 2 threshold 1  2 4
0
 
LVL 45

Assisted Solution

by:Craig Beck
Craig Beck earned 125 total points
Comment Utility
If you don't have those commands you can't map COS to DSCP.

Have a look at this great document which highlights different scenarios.  It will probably explain better than I can :-)

http://www.cisco.com/en/US/products/hw/switches/ps5023/products_tech_note09186a0080883f9e.shtml
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Accepted Solution

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Soulja earned 375 total points
Comment Utility
You don't need those commands in your configuration. When you first enable qos on the switch it will already had default  cos-dscp mapping and vice versa. The same goes for your switched round robin queue. Those command are if you want to customize these settings.

In regards to your phone marking the traffic. It is you need to trust the marking on that interface. Then all you need to do is match the marking in your class map.
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Author Comment

by:leblanc
Comment Utility
That is I wanted to know. I configure those QoS commands if I need to customize my QoS. That is the key.

" all you need to do is match the marking in your class map" or I can just do trust dscp. Correct?
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LVL 26

Assisted Solution

by:Soulja
Soulja earned 375 total points
Comment Utility
I am misunderstanding your statement above. Are you saying that you DO want to customzie your QOS tables?

If you trust dscp, that assumes your phones are tagging  dscp. That way the dscp tag will make it to the interface that you have the policy map on. So you want a class map that matches the tagged traffic.

For example:

class-map test
match dscp EF


Then in your policy map, you can take whatever action you want on that traffic.
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Author Comment

by:leblanc
Comment Utility
Yes. But I can remove the policy-map statement and service-policy input lan-in statement because I already have trust dscp. Correct? My QoS still works. What I am trying to do is to keep things simple,
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LVL 26

Assisted Solution

by:Soulja
Soulja earned 375 total points
Comment Utility
Ah yes. Sorry, misunderstood what you were trying to do. So you just want the tagged traffic to go through being treated by default switch queing than you don't need the policy map on the interface. Trusting the dscp is enough.
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