what is the difference between LLQ and CBWFQ?

[jayaramakrishnan]

As per my understanding in LLQ have dedicated bandwidth and it will use the excess bandwidth if there is no congestion

CEWFQ will use other bandwidth and if there is any congestion then only it will use the reserved bandwidth?

Let's take the example.

Policy Map QOS

    Class VOIP

      priority 10 (%)

      set dscp ef

    Class Call-Signaling

      bandwidth 5 (%)

      set dscp cs3

In the above example:

VOIP will use 10 percet of the total bandwidth. If it exceeds then it may use the other bandwidth.

Call singaling will use remaining bandwidth first if there is any congestion then it use 5% bandwidth.

Whether my understanding is correct or not. Still i am not clear in the CBWFQ.

Need ur advice?

Thanks,

Jayaram

[cadet alain · Accepted solution]

Hi,

VOIP will use 10 percet of the total bandwidth. If it exceeds then it may use the other bandwidth.
Call singaling will use remaining bandwidth first if there is any congestion then it use 5% bandwidth

VoIP will use 10 % of available bandwidth( by default 75% of the bandwidth configured on the interface) and no more in times of congestion.

Call Signaling will get a minimum guaranteed 5% of available bandwidth( same as above) in times of congestion.

These mechanisms are congestion management techniques so it will take place only when there is congestion and LLQ is CBWFQ + priority queue.

Regards.

Alain.

[tj.mitchell · Accepted solution]

The LLQ with the priority will give the voice traffic priority over all the other queues listed. So while it will still reserving the bandwidth, the LLQ will get served first then the other queues.

The rest you have an understanding of..

[jayaramakrishnan]

Thanks cadetalaincadetalain and TJ for your response..

Call Signaling will get a minimum guaranteed 5% of available bandwidth( same as above) in times of congestion.

If there is no congestion in network then it won't use 5% reserved bandwidth first?? during the congestion only it will use the 5% bandwidth?

Pls clarify the same.

Thanks,

Jayaram

[Alen Danielyan · Accepted solution]

The most important info is this:

"Although the bandwidth guarantees provided by the bandwidth and priority commands have been described with words like "reserved" and "bandwidth to be set aside", neither command implements a true reservation. In other words, if a traffic class is not using its configured bandwidth, any unused bandwidth is shared among the other classes.

The queueing system imposes an important exception to this rule with a priority class. As noted above, the offered load of a priority class is metered by a traffic policer. During congestion conditions, a priority class cannot use any excess bandwidth.

So, in case of congestion LLQ traffic cann't use more than is asigned to it, but both can give unused part of own bandwidth to other classes.

For more info see "Comparing the bandwidth and priority Commands of a QoS Service Policy.pdf".

[jayaramakrishnan]

Thanks a lot for your response.. Now i am clear in LLQ and CBWFQ...