Custom queuing functions on the concept of sharing bandwidth among traffic types. Instead of
assigning a priority classification to a specific traffic or packet type, custom queuing forwards
traffic in the different queues by using the FIFO method within each queue. Custom queuing
offers the ability to customize the amount of actual bandwidth that a specified traffic type uses.
While remaining within the limits of the physical line’s capacity, virtual pipes are configured
through the custom queuing option. Varying amounts of the total bandwidth are reserved for various
specific traffic types, and if the bandwidth isn’t being fully utilized by its assigned traffic type,
other types can borrow its bandwidth. The configured limits go into effect during high levels of
utilization or when congestion on the line causes different traffic types to compete for bandwidth.
Figure 30.5 shows each queue being processed, one after the other. After this begins, the
algorithm checks the first queue, processes the data within it, and then moves to the next—if it
comes across an empty one, it will simply move on to the next one without hesitating. Each
queue’s byte count specifies the amount of data that will be forwarded from that queue, which
directs the algorithm to move to the next queue after it has been reached. Custom queuing permits
a maximum of 16 configurable queues. The system queue is for network specific traffic,
including system datagrams such as SNMP and routing updates.
FIGURE 3 0 . 5 Custom queuing algorithm
S0
High priority
(keepalives)
Custom queue
list for S0
20 entries
Default
2
1
0 (System)
Deliver x number
of bytes per cycle
3
14
15
16
Figure 30.6 shows how the bandwidth allocation via custom queuing looks relative to the
physical connection. Using the frame size of the protocols and configuring the byte count for
each queue will configure appropriate bandwidth allocations for each traffic type. In other
words, when a particular queue is being processed, packets are sent until the number of bytes
sent exceeds the queue byte count defined