Difference between priority 500 and bandwidth 500?

[alanchia2000]

Hi,

I'm doing Qos for my network, and have came acrosss 2 terms - priority and bandwidth?

Is there are difference? Does priority means strict scheduling? And what does strict scheduling do?

[vbhadula]

Strict Scheduling will reserverd your bandwidth on the link, no matter if traffic flows or not and that bandwidth is always reserved for the interesting traffic. However bandwidth on the other hand will provide the neccessary bandwidth whenever there is an interesting traffic flow.If there is not interseting traffic then this bandwidth will be used by the other applications.

Stirct priority also lead to the lower queues starvation.

[lferral]

Excuseme I have tha same question, I have specified the priority parameter in one link, and when there is not matches with the traffic specified in the access list, all the traffic is used by other applications, I dont undestand if you say that when you use priority the bandwith specified always will be reserved used or not.

thanks

[alanchia2000]

Hello,

The "priority" command specifies the minimum and maximum bandwidth to use for interesting traffic when a traffic congestion occur, and even when there is no interesting traffic during congestion, the reserved bandwidth cannot be utilised by other traffic.

Perhaps, you can post your config for us to look at.

Alan Chia

[teyobanilom]

"priority" command is used for Low Latency Queueing (LLQ).The BW you specify through this command will provide strict priority queue (always available), usually for delay-sensitive data like voice.

"bandwidth" command is used for Class Based Weighted Fair Queueing. The BW specified with command content for the available BW during congestion.

The BW is in kbps.

HTH

Boyet

[ronald.heitmann]

I think, we should distinguish between 2 kinds of reservation, if we configure LLQ:

---------------

class-map VOIP_RTP

match abcdefgh

class-map VOIP_CTRL

match xyz

policy-map LLQ_FOR_VOIP

class VOIP_RTP

priority XXX_RTP

class VOIP_CTRL

priority XXX_CTRL

class class-default

fair-queue

interface serial 0

bandwidth XXX_START_BANDWIDTH

service-policy output LLQ_FOR_VOIP

-------

a) Bandwidth Reservation to prevent 'overbooking'

We configure a bandwidth on the interface. As default, 75% can be _reserved_ for CBWFQ-Traffic - 25% might be used by the "class-default"-Class. (try "show interface" and see the "available bandwidth"-Counter - it shows the amount of bandwidth wich could be reserved)

Both bandwidths XXX_RTP and XXX_CTRL are reserved - but this is not a QoS-Feature. It just assures, that we can't reserve 2MBit/s on a line wich has 1.5Mbit/s!!!

b) Queuing - Congestion Management

The "VOIP_RTP"-Traffic gets expedited-Forwarding-Behavior - the Traffic will be sent by the router as quick as possible.

The configured priority-"Bandwidth" XXX_RTP is the upper-limit - it configures a "policing"-Feature. (try "priority x ?" on the CLI - you can configure the burst-size of the policer to configure the Tc-duration). This policing-Feature was invented to prevent Queue-Starvation, wich was a problem when we used a feature called "Priority-Queuing" in former times...

So - if there is less VOIP-RTP Traffic than configured, the bandwith will be shared with all "bandwidth"-Queues.

The "VoIP_CTRL"-Traffic gets the configured amount of bandwidth-"bandwidth" XXX_CTRL - if it is needed. If there is "not enough" VOIP_CTRL"-Traffic, the bandwidth can again be used by any other kind of traffic - in the example only the "class-default".

Regards,

//Ronald Heitmann