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QoS on Cat2900XL and 2950

I have some old 2900XL switches running 12.0(5)WC13. Am I right in saying that these do not do WRR? So if I want to do some QoS my best bet is Custom Queueing, right?

How about on my 2950 running 12.1(6)EA2b? I see that those have a command show wrr-queue, so I presume there is some form of WRR, but no way to manipulate it. There seems to be no way of giving a queue priority (tx-queue 3 priority high) as there is in other switches, and no way to see the amount of traffic going through each queue (show int counters detail on other switches).

Anyone got any direct experience of using these features on these switches?

Kevin Dorrell



Re: QoS on Cat2900XL and 2950

For the 2950, you are going to be somewhat limited on the QoS you can deploy. You are limited to 4 queues. The bandwidth per queue can be represented in a percentage. If you would like a priority queue (for voice) then you set the forth queue to a bandwith of 0 to make it the priority queue (will not work on any other queue but the forth queue). You can also map the CoS values (layer two) to the queues that you desire. The 2950 supports an automatic mapping of the CoS to DSCP value (layer 3), but does not follow the standard for voice by default. Cisco phones mark voice with a cos of 5 and call signaling with a cos of 3. The switch will map cos of 5 to DSCP 40. Unfortunately, the best practice is to use DSCP 46 for voice. You have to use a mutation map on the upstream device to correct this. All of these settings are global. Here is an example of setting up a priority queue, setting cos 5 to the priority, and giving weight to each of the other queues:

wrr-queue bandwidth 5 25 70 0

wrr-queue cos-map 1 1

wrr-queue cos-map 2 0

wrr-queue cos-map 3 2 3 4 6 7

wrr-queue cos-map 4 5

You can also force a default CoS value on the switchport with the interface commands of:

mls qos cos

mls qos cos overide

Hope this helps.

Re: QoS on Cat2900XL and 2950

Yes, that does help, thank you. I wonder why the QoS behaviour is different in every switch. The WRR on my 4500s seems to be able to make queue 3 the priority queue, which is what CoS 4 and 5 would go into by default. I have arranged to mark my sensitive traffioc with CoS 5 (it is actually traffic from an IPmux, an E1 circuit emulator that tunnels E1 through IP.)

Now I see that the 2950s allow you to prioritise queue 4, (instead of queue 3), but using a different configuration line. As for the 2900XL, I think I am stuck with custom queue, if it works at all.

I just wish it were a bit more consistent!

Thanks for the answer anyway.

Kevin Dorrell


Re: QoS on Cat2900XL and 2950

I am pretty sure you can't do any sort of configurable QoS on the 2900XL, so Custom Queuing isn't an option. Trunk ports (or Voice VLAN ports which need to be trunks on this platform anyway) will inherantly trust the 802.1p CoS value of received packets but there is no way of manipulating these values. For egress scheduling CoS 0-3 go to a low priority queue and CoS 4-7 go to a high priority queue:



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