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New Member

QoS on catalyst 2950 switches

I have a network that has catalyst 2950 swithes as access switches connected to a 3750 switch that also connects to a 6509 switch as the core.

There are some users connected to the network who have nortel softphones installed on their PCs.The switches have only management vlan and user vlan configured at the access layer.I am very new to this business of VOIP.

Could someone please tell me the exact commands (QoS)that need to be configured on the various switches to give the softphone priority over data traffic.


Re: QoS on catalyst 2950 switches

Which image to you have on the Catalyst 2950s: Standard Image (SI) or Enhanced Image (EI)?

The Standard Image has limited QoS functionality; you may just be able to trust the DSCP value (mls qos trust dscp) which is less than ideal, as the application is being trusted to mark the traffic correctly, and there's nothing to stop a nefarious application from marking all its traffic as 'high priority' and potentially creating a DoS attack.

With the Enhanced Image, MQC/ACL classification is possible. Traffic can be identified at L4 and assigned a DSCP value, and hence a priority. This is preferable to blindly trusting the DSCP values generated by the Nortel Softphones (assuming the soft clients do mark the traffic correctly).

The Priority Queue on the 2950 switch is not enabled unless the WRR-bandwidth for Q4 is set to zero:

mls qos map cos-dscp 0 8 16 26 32 46 48 56

wrr-queue bandwidth 5 25 70 0

wrr-queue cos-map 1 0 1

wrr-queue cos-map 2 2 4

wrr-queue cos-map 3 3 6 7

wrr-queue cos-map 4 5

New Member

Re: QoS on catalyst 2950 switches


I have already configured the mappings and the policies to apply to interfaces. However, I have not found a way to change the queueing strategy from FIFO to WRR. Is there something else missing?