Understand the queuing technologies available in Cisco IOS. You should know that there is
weighted fair queuing (WFQ), priority queuing, and custom queuing. Each has its strengths and
weaknesses, but WFQ is the default technique used for 2.048Mbps or slower interfaces. WFQ can
be tuned with the fair-queue command to enable more conversations to be tracked per interface.
Know how to configure priority queuing and when it is best used. Picking the proper
queuing mechanism is very important, and priority queuing is ideal if you want to ensure that
certain traffic gets priority over other traffic. A priority queue is set up with the prioritylist
command and is applied to the interface with the priority-group command.
Know how to configure custom queuing and when it is best used. Custom queuing is a
technique that enables the WAN designer to allocate a certain amount of bandwidth to different
traffic types. There are 16 queues that can be set up to contain certain types of traffic,
and each of these queues can be allocated a specific amount of bandwidth. A custom queue
is configured with the queue-list command and is applied to the interface with the customqueue-
list command.
Understand compression techniques and algorithms and when to use them. You should know
that the compression techniques are TCP header compression, payload compression, link compression,
and MPPC compression. TCP header compression is used on point-to-point links and
when smaller TCP packets, such as Telnet traffic, are being sent over the link. Payload compression
can be used on links other than point-to-point and is used to compress the data portion of the
packet; it will not alter the layer 2 and layer 3 headers. Link compression is used only on pointto-
point links but is protocol independent. It will compress the whole packet and encapsulate that
packet in another protocol to ensure reliability and sequencing. MPPC is used when the Cisco
device needs to talk to Windows-based clients.
Know the troubleshooting commands used for queuing and compression. The commands to
show queuing are show queue and show queueing [fair | priority | custom]. These can
be used to display queuing on the device and the counters involved. The compression troubleshooting
command is show compress, which can be used to view the compression efficiency and status.