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per.nielsen
Beginner

Cisco 2950 Qos question

On the Cisco 2950 Switch a default CoS-to-DSCP mapping table exists.

How come that 802.1p Cos=5 get's mapped to DSCP=40.

Normally Cos=5 should be mapped to EF DSCP=46

Can anyone explain that wierd mapping ??

Cheers

Per

2 ACCEPTED SOLUTIONS

Accepted Solutions
pkhatri
Advisor

Hi Per,

I believe it is a hardware limitation of the 2950 switch which only allows it to support a small subset of the total number of DSCP values. It is an annoyance and something that needs to be kept in mind when designing end-to-end QoS with 2950 switches in the middle.

The default mapping is a simple multiplication of the CoS value by 8. That's why CoS 5 gives you 40. In DSCP terminology, this would be Class Selector 5 or cs5.

Unfortunately, there is no way to change this to DSCP 46 on the 2950s....

Hope that helps - pls do remember to rate posts that help.

Paresh

View solution in original post

bbaillie
Beginner

The default mapping is what it is in the standard image 2950 switches, unfortunately. The 2950 switches that run "enhanced image" allow the changing of the DSCP-COS and COS-DSCP mappings if you desire the functionality. The details are in a FAQ document.

http://www.cisco.com/en/US/tech/tk389/tk813/technologies_q_and_a_item09186a00804976d3.shtml

Document ID: 46523

Brian

View solution in original post

2 REPLIES 2
pkhatri
Advisor

Hi Per,

I believe it is a hardware limitation of the 2950 switch which only allows it to support a small subset of the total number of DSCP values. It is an annoyance and something that needs to be kept in mind when designing end-to-end QoS with 2950 switches in the middle.

The default mapping is a simple multiplication of the CoS value by 8. That's why CoS 5 gives you 40. In DSCP terminology, this would be Class Selector 5 or cs5.

Unfortunately, there is no way to change this to DSCP 46 on the 2950s....

Hope that helps - pls do remember to rate posts that help.

Paresh

View solution in original post

bbaillie
Beginner

The default mapping is what it is in the standard image 2950 switches, unfortunately. The 2950 switches that run "enhanced image" allow the changing of the DSCP-COS and COS-DSCP mappings if you desire the functionality. The details are in a FAQ document.

http://www.cisco.com/en/US/tech/tk389/tk813/technologies_q_and_a_item09186a00804976d3.shtml

Document ID: 46523

Brian

View solution in original post

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