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leblanc

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3750 QoS

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
...
Avatar of Craig Beck
Craig Beck
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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
Avatar of leblanc
leblanc

ASKER

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
SOLUTION
Avatar of Craig Beck
Craig Beck
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ASKER

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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ASKER

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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