User Manual - Configuration Guide (Volume 3)
Versatile Routing Platform Chapter 3
Congestion Management
3-5
I. FIFO
Dequeuing dispatch
Packets leaving the interface
queue
Packets sent from this interface
Figure QC-3-2 Schematic diagram of FIFO queuing
As shown in the figure above, packets enter FIFO queue in s equence. The dequeuing
sequence is the same as the incoming queue sequence, that is, the packet that arrives
earlier will be sent earlier, while the packet arrives later will be sent later. FIFO does not
make any judgement on the packets, and allocation of network band width and resource
are determined according to the arriving sequence of the packets. T herefore, vicious
application may occupy all network resources, seriously affecting data transm is sion of
key services.
II. PQ
high
medium
normal
low
Classification
Dequeuing dispatch
Packets leaving
the interface
Queue
Packets sent
from this
interface
Figure QC-3-3 Schematic diagram of priority queuing
As shown in the figure above, PQ is used to provide strict prior ity for important network
data. It ensures that the most important data in the network unit using PQ can be
processed as fast as possible. PQ can flexibly specify the priority sequence according
to network protocol (for example, IP and IPX), data inflow interface, length of the packet
and source address/destination address.
When the queues are dispatched, PQ first sends the packets in the queue with hi gher
priority in strict compliance with high-to-low sequence. If the queue with hig her pr ior it y
is empty, the packets in the queue with lower priority can be sent. The packets in the
queue with lower priority might be congested due to the existence of packets in the
queue with higher priority. Therefore, the packets of key service (for example, ERP) are
put in the queue with higher priority, and the packets of ordinary service (for example,
E-mail) are put in the queue with lower priority. In this way, it can be ensured that
packets of key services are transmitted with priority, and the packets of ordinary
services are transmitted in the idle intervals during the processing of the k ey service
data.