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New Member

7600 QoS Strict-Priority


I'm testing in a LAB QoS configuration for 7600 (SUP720-3B) with 12.2-18.SXF3 with many diffenents cards.

Following Cisco documentation, I have two diferent scenarios:

1. OSM and WAN interfaces supports CBWFQ and LLC and it works fine, as I understand it should work. We could create a priority queue and many standard queues to clasify the traffic and assign differents BW.

2. LAN interfaces don't support CBWFQ and LLC and we should use the Queue Architecture of the card and WRR and WRED, which are implemented on the ASICs. This is what Cisco calls Hardware-Switched Traffic.

Here it depends of the card you use what Queue Architecture you could use. But most cards have a Strict-Priority Queue and one or more standard queues.

My question is regarding about the diferent behavior of the priority queue (LLC) and the Strict-Priority queue in case of congestion on the interface.

From my testings I got:

+ 1st scenario (Priotity Queue - LLC)

In case of congestion of the interface, QoS provide to the Priority Queue just the BW configured. If the BW offered in this queue is higher than the BW configured, there're packet drops.

The rest of the standard queues shares the rest of the BW available on the interface.

+ 2nd scenario (Hardware-Switched Traffic)

Even with or without congestion, I get almost the same behavior. The router always try to transmit the traffic of the strict-priority queue first. So it doesn't matter how much BW you allocate to the strict-priority queue and the standard queues. The card always try to transmit the traffic of strict-priority queue first.

Is this the desired behavior or there's something wrong?

The behavior of the priority queue should be the same in both scenarios?

I got the following answers from Cisco Web Site which doesn't answer my question completely (See attached file).

Anyone tried this before???



New Member

Re: 7600 QoS Strict-Priority

If you have not visited the following document, just refer the same. Because, this document clearly explains all queuing techniques.

New Member

Re: 7600 QoS Strict-Priority

Hello Baley,

Thanks for your reply. I read this link before but it doesn't explain exactly what happens in the scenario described before.

Few days ago I got an answer from Cisco which confirms what I read from the FAQ: Till Strict-Priority Queue isn't empty, packets on the WRR queues aren't send. This behavior is completely different from the well known LLQ.

Catalyst 6500/6000 QoS FAQ

Q. On egress queuing, if the strict priority queue is saturated, will traffic eventually be served in the weighted round-robin (WRR) queues?

A. No, the WRR queues are not served until the priority queue is completely empty.