QOS on trusted ports.

peter.scott · ‎03-27-2003

After reading a lot of literature on QOS, I was wondering if somebody could elaborate on step 3 in the following, with regard to the Internal COS label.

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Layer 2, trust based on ingress port

1. If the port is untrusted, apply the default port CoS and switch the frame.

2. If the frame is not trunked, apply the port CoS but continue with the next step.

3. If the port is set to trust IP Precedence, use the IP Precedence (apply the Precedence as internal CoS label) and switch the frame.

4. If the port is set to trust DSCP, do similarly.

5. If the port is set to trust CoS, queue the frame, using receive queues and thresholds based on the hardware and configured drop thresholds (see the previous article).

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Does this imply that if Trust-Prec or Trust-DSCP is used then the frame would not use the RX Priority Queue?

The scenario that I have at the moment is,

A VG248 that is marking rtp traffic with DSCP 46 connected to a port on a 3524.

On another port I have LRE2924 with IP phones hanging of that. The ip phones are stamping RTP traffic with a COS value of 5 and a DSCP value of 46.

The 6500 port that's connected to the 3524 is configured to Trust-DSCP.

Many Thanks

dgoodwin · ‎03-27-2003

Hi Peter,

The 3524 doesn't change any QoS markings, it simply prioritizes packets based on the observed COS values. So from the 6500's perspective, pretend it's not there for the purposes of QoS.

In your specific case, if the only device connected to the 3524 is the VG248, then you should be able to Trust-DSCP on the 6500 port connected to the 3524, since there would be nothing else contending for space.

However, assuming you have some other devices attached to the same 3524, the most comprehensive way to set QoS on the 6500 port is through a QoS ACL. This URL should get you started:

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/sw_6_3/confg_gd/qos.htm

If you are running native IOS mode, I think this URL should help:

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/12111bex/swcg/qos.htm

peter.scott · ‎03-27-2003

Thanks for the reply.

I need to run Trust-DSCP on the Cat6500, because there is a number of different devices attached to the 3524.

My question is, if I trust the port with Trust-COS, then i'm led to believe that the frame will be put into the RX Priority Queue if there is one.

If i trust the port with Trust-DSCP or Trust-Precedence, then I'm led to believe that the precedence or DSCP will be used as an internal Cos value.

Is this implying that the frame will be put into the RX Priority Queue or not?

If not, what RX queue will it be placed into?

Extract from a document that I found.