Solved: question about 3750 mls qos map dscp-output-q and cos-output-q

richard.gu · ‎11-10-2007

1. If a egress packet has both Cos and Dscp setting, which map should this packet used to put into queue?

2. The 3750 is doing ip route. After the packet is routed, will the packet keep the DSCP and COS? Or it will just keep the DSCP and using the dscp-cos map to create a new COS.

allan.thomas · ‎11-10-2007

Apologies for the confusion with the terminalogy.

The question is where you have configured the trust boundaries, do you necessarily trust the DSCP value prior to being routed across your network?

Therefore, although you have explicity trusted the DSCP value, do you still trust the value at the remote peer.

For example, you connect into an MPLS with QoS enabled, you know that the values you are trusting are correct within your network, however at the remote peer/branch they could be remarked by the provider. Therefore do you 'believe' the values, or do you simply not trust them and then reclassify on ingress.

Regards

Allan.

Hope this makes sense..

View solution in original post

allan.thomas · ‎11-10-2007

The COS will only be remarked if the DSCP value is not trusted on ingress. Remember where your trust boundaries are configured.

If the COS is trusted then when the packet is encapsulated with L3 header then the COS will be unaffected when it routed.

The issue will be whether you trust value on egress, if the port is untrusted the DSCP will be re-written and subsequently the COS will be also.

Span the ports on ingress and egress and analyse the output you should be able see the dscp and cos value within each frame.

Regards

Allan.

richard.gu · ‎11-10-2007

How can I configure trust value on egress?

allan.thomas · ‎11-10-2007

Apologies for the confusion with the terminalogy.

The question is where you have configured the trust boundaries, do you necessarily trust the DSCP value prior to being routed across your network?

Therefore, although you have explicity trusted the DSCP value, do you still trust the value at the remote peer.

For example, you connect into an MPLS with QoS enabled, you know that the values you are trusting are correct within your network, however at the remote peer/branch they could be remarked by the provider. Therefore do you 'believe' the values, or do you simply not trust them and then reclassify on ingress.

Regards

Allan.

Hope this makes sense..