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Community Member

QoS critique needed

Hey all, I was wondering if you could critique my QoS config?? MY topology is three sites connected via MPLS. Each connection is only a T1. Each site has its own separate gateway for Internet access so only traffic destined between each site is passed into the cloud. Here is an image of how everything is architected as well as a partial QoS config. This snapshot is from the access-layer. Since best practice is to mark traffic as close to the source as possible I have chosen to implement ingress service-policies on each access-port. Thoughts??


Hall of Fame Super Silver

Re: QoS critique needed

Hello Unclerico,

you should find a way to limit the number of concurrent Voip calls. This can be done on the IP PBX controlling the phones.

if C2960 supports marking using policy-map based on IP ACLs you should be fine.

>> To use policing and marking, the switch must be running the LAN Base image.

configuration with policy maps is possible so this point should be fine.

on the wan router you need to define a scheduler, you should use a priority queue for CS5, bandwidth commands for the other classes.

Hope to help


Community Member

Re: QoS critique needed

Thank you for your feedback Guiseppe, as always!!!

Re: QoS critique needed

Hi unclerico,

Does All Access switches perform layer-3 functionality?

At the Access layer, you should mark based on CoS values not dscp.

Your configuration is fine but the marking should be done based on the CoS, and then you will need to map the layer 2 marking to layer 3 marking at the core.

Finally, you will apply the QoS policy outbound to the T1 towards the service provider.



Community Member

Re: QoS critique needed

Mohammed, thank you for your response. Can you please let me know why it would be beneficial to mark CoS rather than ToS at the access layer?? Thanks.

Re: QoS critique needed


That's Why I asked if the switch performs layer-2 or layer-3 switching.

If its layer-2, then the switch doesnt look into the ip header, the 1 byte TOS field is part of the IP header and therfore all your marking is useless.

If The interface the hosts connected to is layer-3 interface then , I would say yes, you can mark based on DSCP.

The Cos is part of the layer 2 header and the layer2 switch can understand any marking done at layer2 and performs its action and policy according to the layer-2 marking.



Re: QoS critique needed

what L2 switch are you using ?

if you trust DSCP in your access port  then the switch will use the dscp-cos mapping in the switch nothing will be lost but you need to be aware about dscp-cos mapping and cos to dscp mapping

unless the traffic is using non-IP traffic in this case you need to use COS which is not part of the case above

if you have 3560 switch you may trust cos in the port but pass the DSCP as it is without rewriting it based on the cos-DSP mapping

by using the foolwing command:

no mls qos rewrite ip dscp

good luck

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