Tag Archives: QoS

Trust DSCP is a must have on Cisco Catalyst 4500

This is a small post about one specific Cisco Catalyst 4500 Classification characteristic. The one I was lucky to find and read in advance before I deployed QoS configuration changes. Here it is.

If you run Cisco Catalyst 4500 on a Supervisor card older than V-10GE, software older than Cisco IOS Release 12.2(31)SG and you classify traffic with service-policy that refers to DSCP value (via ACL, or class-map match statements), you MUST HAVE qos trust dscp configured on a port! 

Simple as that. If you don’t do that, your service-policy won’t see DSCP values, and won’t be able to classify traffic in a right manner. Here’s a screenshot from the Cisco Catalyst 4500 QoS configuration guide that proves my words.



Cisco IOS Management Plane Default QoS

Just an observation.

Cisco IOS routers and switches mark locally originated SSH and Telnet traffic with 802.1p = 6, DSCP = CS6 (48). That is, if you initiate an SSH session to the router, the returning traffic will have DSCP set to CS6. It’s a bit weird because Cisco IOS CLI states that default DSCP value for locally originated SSH packets is 0. Continue reading

Verify port-based QoS configuration in hardware of Cisco 6509

It’s a pleasure to work with Cisco TAC. I would say this is the only way to instantly improve your troubleshooting skills in any area. I was involved in Cisco 6509 troubleshooting that turned to be an IOS software-to-hardware QoS configuration bug. With this post I will slightly cover the bug logic and will show you the way to confirm QoS configuration in Cisco 6509’s module hardware.
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