QoS Question ?

[Hieu Cao]

Our network router is currently configured to use QoS (CBWFQ with LLQ) as we do use VoIP. In our router config, there's following statement that I've a couple of questions about that I hope that someone can help to shed some lights..

class-map match-all Match-All-VC

match access-group 115

class-map match-all VoIP-RTP

match access-group 110

policy-map QoS-Policy-T1

class VoIP-RTP

priority 512

class Match-All-VC

priority 512

class class-default

fair-queue

access-list 110 permit ip any any precedence critical

access-list 110 permit ip any any dscp ef

access-list 110 permit tcp any eq 1720 any

access-list 110 permit tcp any any eq 1720

access-list 110 permit ip any any precedence flash

access-list 115 permit tcp any any precedence flash-override

Question:

A. How many priority queues are allowed per serial interface? The above policy is applied to all of our serial interfaces.

B. What is the purpose of the command "precedence flash" and "precedence flash-override"? I believe that their priorities level are 4 and 3 respectively, but I am not sure of what it does.

TIA..

Hieu

[pkhatri]

Hi,

A. There is only one priority queue per interface. When you configure the "priority" statement for multiple classes, all of the traffic for those classes ends up in a common priority queue.

B. "flash" and "flash-override" were some of the original names given to the precedence bits for the IP protocol. Check out http://www.ietf.org/rfc/rfc791.txt. There is absolutely no issues with referring to them with their numbers.

Hope that helps - pls do rate the post if it does.

Paresh

[Hieu Cao]

Thanks Paresh for your response!

Hieu

[Hieu Cao]

Paresh,

Since we're attempting to separate both voice and data traffic into separate queues, shouldn't we have one Priority statement for VoIP? If we've both voice & data in the same priority queue as shown in our class-map/policy, is there any benefit or drawback? I don't think it's efficient to combine both voice & data in the same priority queue, so please let me know what your thoughts are..

Thanks!

Hieu

[pkhatri]

Hi Hieu,

That is absolutely correct. It is poor design to have both voice and data in the same queues.

What you should really do is to assign a 'bandwidth' statement to your Match-All-VC class. That way, your VoIP traffic will get absolute priority. And the Match-All-VC will get a bandwidth guarantee without impacting the VoIP traffic.

Paresh

[Hieu Cao]

Hello Paresh,

Thank you very much for your feedback!

Hieu