LLQ and bandwidth restrictions

[reinke]

I have configured CBWFQ and I am using one strict priority queue. The queue should get bandwidth x (priority x). Here is the question:

This bandwidth is guaranteed, but is it limited to x as well. E.g. the outgoing link is not congested:

- High priority traffic is not limited to x?

or

- High priority traffic is still limited to x?

The last one is what I hope to get. If this statement is not true, do I have to use rate-limiting in combination with the strict priority queue?

Thanks in advance

Edgar

[rabeder · Accepted solution]

hi,

hope this helps:

http://www.cisco.com/en/US/customer/products/sw/iosswrel/ps1830/products_feature_guide09186a0080087b13.html

Guaranteed Bandwidth

When you specify the priority command for a class, it takes a bandwidth argument that gives maximum bandwidth in kilobits per second (kbps). You use this parameter to specify the maximum amount of bandwidth allocated for packets belonging to the class configured with the priority command. The bandwidth parameter both guarantees bandwidth to the priority class and restrains the flow of packets from the priority class.

In the event of congestion, when the bandwidth is exceeded policing is used to drop packets. Voice traffic enqueued to the priority queue is UDP-based and therefore not adaptive to the early packet drop characteristic of Weighted Random Early Detection (WRED). Because WRED is ineffective, you cannot use the WRED random-detect command with the priority command. In addition, because policing is used to drop packets and queue limit is not imposed, the queue-limit command cannot be used with the priority command.

When congestion occurs, traffic destined for the priority queue is metered to ensure that the bandwidth allocation configured for the class to which the traffic belongs is not exceeded.

Priority traffic metering has the following qualities:

It is much like Committed Access Rate's (CAR) rate limiting, except that priority traffic metering is only performed under congestion conditions. When the device is not congested, the priority class traffic is allowed to exceed its allocated bandwidth. When the device is congested, the priority class traffic above the allocated bandwidth is discarded.

It is performed on a per-packet basis, and tokens are replenished as packets are sent. If not enough tokens are available to send the packet, it is dropped.

It restrains priority traffic to its allocated bandwidth to ensure that nonpriority traffic, such as routing packets and other data, is not starved.