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When to change the 'mls qos map cos-dscp'?

I am following the 'Cisco Catalyst 6500 Voice Deployment Guide' to configure up 6509s as access-layer devices for all our IP phones and PC users. We'll be combining the PC into the IP phone into a switchport, and using Avaya IP Phones.

We will be working with DSCP and not COS and the Avaya IP phones will mark their voice and control packets to 46 and 24 DSCP respectively.

Here is an example config of one of the switch ports:

interface GigabitEthernet2/1

description IP phone and PC

switchport access vlan 60

switchport voice vlan 910

srr-queue bandwidth share 10 10 60 20

srr-queue bandwidth shape 10 0 0 0

mls qos trust dscp

auto qos voip trust

spanning-tree portfast

My question has to do with the default 'mls qos map cos-dscp' which is 0 8 16 24 32 40 48 56.

Since I'm working with DSCP only, do I need to change the cos-dscp map? Is it used when I am trusting DSCP?

In some examples I'm seen it is changed to mls qos map cos-dscp 0 8 16 24 32 46 48 56...

...but that is in examples where 'mls qos trust cos' is being used.

I'm confused about that part of the config and appreciate any insight you have.

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Green

Re: When to change the 'mls qos map cos-dscp'?

Hi,

"In some examples I'm seen it is changed to mls qos map cos-dscp 0 8 16 24 32 46 48 56... "

...but that is in examples where 'mls qos trust cos' is being used.

When QoS is enabled on the switches, the switch configures a series of default QoS mappings. In its default mappings, the traffic that is marked with CoS 5 and ToS (IPP bit value) 5 is mapped to a DSCP value of 40, and traffic that is marked with CoS or Tos 3 is mapped to a DSCP value of 24.

The 'mls qos map cos-dscp' changes this setting.

If you are using 6500 with PFC, you should be aware that the PFC provides the capability to handle classification and marking in the IP Packet at Layer 3, instead of being limited to the Layer 2 CoS values learned from 802.1p. When you are working with 6500 without a PFC or any Layer 2 switch that can perform only Layer 2 CoS marking, you have to rely on distribution and core layer switches that can map Layer 2 Cos to Layer 3 ToS values accurately.

If you configure the trust trust-dscp command will be enough since IP Phones mark the voice-beare and signalling appropiatly using both CoS and DSCP map.

HTH

1 REPLY
Green

Re: When to change the 'mls qos map cos-dscp'?

Hi,

"In some examples I'm seen it is changed to mls qos map cos-dscp 0 8 16 24 32 46 48 56... "

...but that is in examples where 'mls qos trust cos' is being used.

When QoS is enabled on the switches, the switch configures a series of default QoS mappings. In its default mappings, the traffic that is marked with CoS 5 and ToS (IPP bit value) 5 is mapped to a DSCP value of 40, and traffic that is marked with CoS or Tos 3 is mapped to a DSCP value of 24.

The 'mls qos map cos-dscp' changes this setting.

If you are using 6500 with PFC, you should be aware that the PFC provides the capability to handle classification and marking in the IP Packet at Layer 3, instead of being limited to the Layer 2 CoS values learned from 802.1p. When you are working with 6500 without a PFC or any Layer 2 switch that can perform only Layer 2 CoS marking, you have to rely on distribution and core layer switches that can map Layer 2 Cos to Layer 3 ToS values accurately.

If you configure the trust trust-dscp command will be enough since IP Phones mark the voice-beare and signalling appropiatly using both CoS and DSCP map.

HTH

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