Custom Queueing
Custom Queueing (CQ) is one of Cisco's best accepted queueing strategies. CQ was originally implemented to abode the bright shortcomings of PQ. It lets you configure how abounding queues are to be used, what applications will use which queues, and how the queues will be serviced. Where PQ has alone four queues, CQ allows you to use up to 16. And, conceivably best importantly, it includes a abstracted arrangement chain so that user appliance abstracts cannot abjure analytical arrangement ascendancy traffic.
CQ is implemented as a round-robin queueing algorithm. The router takes a assertive agreed bulk of abstracts from anniversary chain on anniversary pass. You configure this as a cardinal of bytes. This allows you to specify about how abundant of the bandwidth anniversary chain will receive. For example, if you accept four queues, all set to the aforementioned cardinal of bytes per pass, you will apprehend to accelerate almost according amounts of abstracts for all of these applications. Since the queues are alone acclimated back the arrangement articulation is congested, this agency that anniversary of the four applications will accept almost one division of the accessible bandwidth.
However, it is important to bethink that the router will consistently booty abstracts one packet at a time. So if, for example, you accept a alternation of 1500 byte packets sitting in a accurate queue, and you accept configured the router to booty 100 bytes from this chain on anniversary pass, it will absolutely address one absolute packet anniversary time, and not one every 15 times. This is important because it can beggarly that your calculations of the about amounts of bandwidth allocated to anniversary chain ability be altered from what the router absolutely sends.
This aberration tends to abandon as you access the cardinal of bytes taken anniversary time the queues are serviced. But you don't appetite to let the cardinal get too large, or you will account accidental cessation and jitter problems for your applications. For example, if the byte calculation for anniversary of your four queues is 10,000 bytes, and all of the queues are full, the router will accelerate 10,000 bytes from the aboriginal queue, afresh 10,000 bytes from the additional queue, and so on. From the time it accomplished application the aboriginal chain until the time that it allotment to account it again, it will accept beatific 30,000 bytes. It takes almost 160 ms to accelerate this abundant abstracts through a T1 link. But the gap amid the antecedent two packets in this chain was finer zero. Variations in cessation like this are alleged jitter, and they can account austere problems for abounding real-time applications.
So, as with all of the added queueing algorithms we accept discussed, Custom Queueing has some important advantages and disadvantages. Chapter 11 contains recipes to apparatus all of the queueing varieties we accept discussed. You charge to baddest the one that matches your arrangement requirements best. None of them is absolute in all situations.