background image
306 Chapter 9: WAN Design Considerations
The following example assigns a medium-priority level to every AppleTalk packet that has a
size greater than 200 bytes:
priority-list 2 protocol apple medium gt 200
The following example assigns a high-priority level to traffic that matches IP access list 20:
priority-list 1 protocol ip high list 20
The following example assigns a low-priority level to Telnet packets:
priority-list 4 protocol ip low tcp 23
Figure 9-9 illustrates priority queuing. When a packet is to be sent out an interface, the priority
queues on that interface are scanned for packets in descending order of priority. The high-
priority queue is scanned first, then the medium-priority queue, and so on. The packet at the
head of the highest queue is chosen for transmission. This procedure is repeated every time a
packet is to be sent. The maximum length of a queue is defined by the length limit. When a
queue is longer than the queue limit, all additional packets are dropped.
Custom Queuing
Custom queuing allows for a fairer approach to queuing. With custom queuing, every traffic or
packet type has a chance of receiving at least a minimum level of service. Each queue is
serviced in sequence. A percent of traffic in each queue is passed before the next queue is
processed. Custom queuing guarantees that mission-critical data is always assigned a certain
percentage of the bandwidth, but it also ensures predictable throughput for other traffic. The
amount of bandwidth can be configured and reserved for each traffic type.
Custom queuing prioritizes multiprotocol traffic and allows for a maximum of 16 queues to be
built. Each queue is serviced sequentially until the number of bytes sent exceeds the
configurable byte count or the queue is empty. Custom queuing is designed for environments
that want to ensure at least a bare minimum of service for all protocols. It allows protocols of
different characteristics to share the media.
Figure 9-9
Priority Queuing
Arriving Packet
Serial 0
Router compares
arriving packet with
priority lists and
marks the packet
Priority list for serial 0
Queue 1
Queue 2
Queue 3
Queue 4
High
Medium
Normal
Low
87200333.book Page 306 Wednesday, August 22, 2001 2:53 PM