end-to-end Qos

Unanswered Question
Mar 27th, 2008

This is my scenario and I want to do end-to-end VoIP QoS. My L2 is a Cisco 3750, L3 being Cisco 6500 and the routers are Cisco 3825

desktop --> phone --->L2 ----> trunk ------> core ----> router ---> router ---> core ---> trunk ---> L2 --> phone --> desktop

Let me know whether my understanding is right

1) On the L2 trust DSCP

2) the switch will do a CoS-DSCP marking

3) on the trunk, VoIP will ve given priority

4) on the L3, write a policy to mark the VoIP network and the relevant voice ports with EF

5) on the L3, write a policy to mark the VoIP network and the relevant voice signalling ports with CS3

6) On the L3, write a policy to mark all other traffic as Best effort.

7) On WAN, trap the EF and CS3 traffic and allocate bandwidth.

Is my logic right?


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royalblues Thu, 03/27/2008 - 03:35


Marking should be done as close to the source as possible. So mark your traffic at the L2 and trust those marking at the L3

In addition to the above, you shoudl tune your interface queues and assign voice to priority queue



saimbt Thu, 03/27/2008 - 03:50

Hi Narayan,

On the L2, how will I trap the traffic?


saimbt Fri, 03/28/2008 - 11:26

there is a slight twist. after the L3--> WAN switch --> rtr

what should i be doing on the SVI interface on the L3?


royalblues Sat, 03/29/2008 - 00:34

It depends whether you want to do vlan Qos or port level QOS

When you do port level QoS, all you need to ensure is that the trunk ports are configured to trust the markings and interface queues are properly configured


saimbt Sat, 03/29/2008 - 10:54

the link between the L3 and WAN switch is a L2 link and not a trunk. similarly the link between the wan swittch and rtr is again a l2 link



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